我试图使用Cmake将一个外部库安装到我的C++项目中。我希望使用该库生成Xcode项目。在我的终端中，我从build目录运行以下代码：

cmake -G Xcode ..

这会产生以下错误：

-- The CXX compiler identification is unknown
-- The C compiler identification is unknown
CMake Error at CMakeLists.txt:6 (project):
No CMAKE_CXX_COMPILER could be found.

CMake Error at CMakeLists.txt:6 (project):
  No CMAKE_C_COMPILER could be found.

我使用的是g++编译器：

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
    Apple LLVM version 7.0.0 (clang-700.1.76)
    Target: x86_64-apple-darwin14.5.0
    Thread model: posix

编辑：CMakeLists.txt文件

file(READ "${CMAKE_CURRENT_SOURCE_DIR}/VERSION" PROJECT_VERSION_FULL)
string(REGEX REPLACE "[\n\r]" "" PROJECT_VERSION_FULL "${PROJECT_VERSION_FULL}")
string(REGEX REPLACE "^([0-9]+)\\.[0-9]+\\.[0-9]+$" "\\1" PROJECT_VERSION_MAJOR "${PROJECT_VERSION_FULL}")
string(REGEX REPLACE "^[0-9]+\\.([0-9]+)\\.[0-9]+$" "\\1" PROJECT_VERSION_MINOR "${PROJECT_VERSION_FULL}")
string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+)$" "\\1" PROJECT_VERSION_PATCH "${PROJECT_VERSION_FULL}")
set(PROJECT_VERSION "${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}")
math(EXPR LIBRARY_VERSION_MAJOR "${PROJECT_VERSION_MAJOR}")
set(LIBRARY_VERSION_MINOR "${PROJECT_VERSION_MINOR}")
set(LIBRARY_VERSION_PATCH "${PROJECT_VERSION_PATCH}")
set(LIBRARY_VERSION "${LIBRARY_VERSION_MAJOR}.${LIBRARY_VERSION_MINOR}")
set(LIBRARY_VERSION_FULL "${LIBRARY_VERSION}.${LIBRARY_VERSION_PATCH}")

option(CODE_COVERAGE "Set ON to add code coverage compile options" OFF)
option(GENERATE_DOC "Set ON to genrate doxygen API reference in build/doc directory" OFF)

# C++11 compiler Check
if(NOT CMAKE_CXX_COMPILER_VERSION) # work around for cmake versions smaller than 2.8.10
    execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpversion OUTPUT_VARIABLE CMAKE_CXX_COMPILER_VERSION)
endif()
if(CMAKE_CXX_COMPILER MATCHES ".*clang" OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
      set(CMAKE_COMPILER_IS_CLANGXX 1)
endif()
if( (CMAKE_COMPILER_IS_GNUCXX AND ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 4.7) OR
    (CMAKE_COMPILER_IS_CLANGXX AND ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 3.2))
  message(FATAL_ERROR "Your C++ compiler does not support C++11. Please install g++ 4.7 (or greater) or clang 3.2 (or greater)")
else()
  message(STATUS "Compiler is recent enough to support C++11.")
endif()

if( CMAKE_COMPILER_IS_GNUCXX )
    append_cxx_compiler_flags("-std=c++11 -Wall -Wextra  -DNDEBUG" "GCC" CMAKE_CXX_FLAGS)
    append_cxx_compiler_flags("-O3 -ffast-math -funroll-loops" "GCC" CMAKE_CXX_OPT_FLAGS)
    if ( CODE_COVERAGE )
        append_cxx_compiler_flags("-g -fprofile-arcs -ftest-coverage -lgcov" "GCC" CMAKE_CXX_FLAGS)
    endif()

else()
    if(MSVC)
        append_cxx_compiler_flags("/EHsc" "MSVC" CMAKE_CXX_FLAGS)
        append_cxx_compiler_flags("/Od" "MSVC" CMAKE_CXX_FLAGS_DEBUG)
        append_cxx_compiler_flags("/Ox" "MSVC" CMAKE_CXX_FLAGS_RELEASE)
        set(vars CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
        foreach(var ${vars})
            string(REPLACE "/MD" "-MT" ${var} "${${var}}")
        endforeach(var)

        add_definitions("/DMSVC_COMPILER")
    else()
        append_cxx_compiler_flags("-std=c++11 -DNDEBUG" "CLANG" CMAKE_CXX_FLAGS)
        append_cxx_compiler_flags("-stdlib=libc++" "CLANG" CMAKE_CXX_FLAGS)
        append_cxx_compiler_flags("-O3 -ffast-math -funroll-loops" "CLANG" CMAKE_CXX_OPT_FLAGS)
    endif()
endif()

也可以使用CMakeEdit.log：

Compiling the CXX compiler identification source file "CMakeCXXCompilerId.cpp" failed.
Compiler:  
Build flags: 
Id flags: 

The output was:
1
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

Compiling the C compiler identification source file "CMakeCCompilerId.c" failed.
Compiler:  
Build flags: 
Id flags: 

The output was:
1
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

编辑：

当xcode-select开发者目录指向/Library/Developer/CommandLineTools时，如果需要完整的常规XCode（在XCode之后安装CLT时会发生此问题），则会发生此问题。
我找到的解决办法是这样的：

sudo xcode-select -s /Applications/Xcode.app/Contents/Developer

完成此操作后，当我运行cmake -G Xcode ..时，我得到其他错误：

-- The CXX compiler identification is AppleClang 7.0.0.7000176
-- The C compiler identification is AppleClang 7.0.0.7000176
CMake Error at /usr/local/Cellar/cmake/3.4.0/share/cmake/Modules/Platform/Darwin.cmake:76 (message):
  CMAKE_OSX_DEPLOYMENT_TARGET is '10.10' but CMAKE_OSX_SYSROOT:

   ""

  is not set to a MacOSX SDK with a recognized version.  Either set
  CMAKE_OSX_SYSROOT to a valid SDK or set CMAKE_OSX_DEPLOYMENT_TARGET to
  empty.
Call Stack (most recent call first):
  /usr/local/Cellar/cmake/3.4.0/share/cmake/Modules/CMakeSystemSpecificInformation.cmake:36 (include)
  CMakeLists.txt:6 (project)

EDIT 2操作系统指定的SDK似乎错误。

CMAKE_OSX_DEPLOYMENT_TARGET is '10.10' but the matching SDK does not exist
  at:

   "/Applications/DEV/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk"

  Instead using SDK:

   "/Applications/DEV/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk".
Call Stack (most recent call first):
  /usr/local/Cellar/cmake/3.4.0/share/cmake/Modules/CMakeSystemSpecificInitialize.cmake:18 (include)
  CMakeLists.txt:6 (project)

-- The CXX compiler identification is AppleClang 7.0.0.7000176
-- The C compiler identification is AppleClang 7.0.0.7000176
CMake Error at /usr/local/Cellar/cmake/3.4.0/share/cmake/Modules/Platform/Darwin.cmake:76 (message):
  CMAKE_OSX_DEPLOYMENT_TARGET is '10.10' but CMAKE_OSX_SYSROOT:

   ""

  is not set to a MacOSX SDK with a recognized version.  Either set
  CMAKE_OSX_SYSROOT to a valid SDK or set CMAKE_OSX_DEPLOYMENT_TARGET to
  empty.
Call Stack (most recent call first):
  /usr/local/Cellar/cmake/3.4.0/share/cmake/Modules/CMakeSystemSpecificInformation.cmake:36 (include)
  CMakeLists.txt:6 (project)