在 MinGW 和 MinGW-64 上执行命令后变量为空?

我们正在为在MinGW和MinGW-64上失败的CMake构建而苦苦挣扎。我们的项目是一个C++库。我们的CMakeLists.txt有一个如下所示的块。该块通过编译器识别目标平台(其他方法太不可靠，尤其是在 ARM、MIPS 和 PPC 上(：

set(SHELL_CMD sh -c)
set(GREP_CMD egrep -i -c)
execute_process(COMMAND ${SHELL_CMD} "${CMAKE_CXX_COMPILER} -dumpmachine 2>&1"
COMMAND ${GREP_CMD} "amd64"
OUTPUT_VARIABLE CRYPTOPP_AMD64
OUTPUT_STRIP_TRAILING_WHITESPACE)
...
# http://github.com/weidai11/cryptopp/issues/466
execute_process(COMMAND ${SHELL_CMD} "${CMAKE_CXX_COMPILER} -dumpmachine 2>&1"
COMMAND ${GREP_CMD} "mingw32"
OUTPUT_VARIABLE CRYPTOPP_MINGW32
OUTPUT_STRIP_TRAILING_WHITESPACE)
# http://github.com/weidai11/cryptopp/issues/466
execute_process(COMMAND ${SHELL_CMD} "${CMAKE_CXX_COMPILER} -dumpmachine 2>&1"
COMMAND ${GREP_CMD} "w64-mingw32"
OUTPUT_VARIABLE CRYPTOPP_MINGW64
OUTPUT_STRIP_TRAILING_WHITESPACE)

它在11个平台中的9个平台上成功，但在MinGW和MinGW-64上失败。从我们跟踪该问题的错误报告中，MinGW-64 上的以下内容：

execute_process(COMMAND ${SHELL_CMD} "${CMAKE_CXX_COMPILER} -dumpmachine 2>&1"
COMMAND ${GREP_CMD} "w64-mingw32"
OUTPUT_VARIABLE CRYPTOPP_MINGW64
OUTPUT_STRIP_TRAILING_WHITESPACE)
message(STATUS ${CRYPTOPP_MINGW64})
execute_process(COMMAND ${SHELL_CMD} "${CMAKE_CXX_COMPILER} -dumpmachine 2>&1"
COMMAND ${GREP_CMD} "mingw32"
OUTPUT_VARIABLE CRYPTOPP_MINGW32
OUTPUT_STRIP_TRAILING_WHITESPACE)
message(STATUS ${CRYPTOPP_MINGW32})

这些语句生成一条空消息。删除OUTPUT_STRIP_TRAILING_WHITESPACE不会影响问题。从MinGW终端运行c++ -dumpmachine | egrep -i -c w64-mingw32会产生预期的结果。

我们发现CMAKE_COMPILER_IS_GNUCXX和CMAKE_CXX_COMPILER_ID是空的，但它似乎不适用，因为编译器在我们的例子中设置。(这是我们找到的唯一另一个"cmake 空变量 mingw"(。

不幸的是，我不知道正在使用哪个版本的Cmake。我们的错误报告包括 CMake 输出，但 CMake 无法打印其版本号。

为什么CMake无法在MinGW和MinGW-64上设置变量CRYPTOPP_MINGW64和CRYPTOPP_MINGW32？

下面是配置时 CMake 的典型输出。

=== Building cryptopp ===
mkdir -p deps/cryptopp/build/
cd deps/cryptopp/build/ && cmake -G 'MSYS Makefiles' -D CMAKE_BUILD_TYPE=Release -D BUILD_TESTING=OFF -D BUILD_SHARED=OFF -DCMAKE_CXX_FLAGS="-march=native" ../ && make
-- The C compiler identification is GNU 6.2.0
-- The CXX compiler identification is GNU 6.2.0
-- Check for working C compiler: C:/msys32/mingw32/bin/gcc.exe
-- Check for working C compiler: C:/msys32/mingw32/bin/gcc.exe -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: C:/msys32/mingw32/bin/g++.exe
-- Check for working CXX compiler: C:/msys32/mingw32/bin/g++.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - found
-- Found Threads: TRUE  
-- Compiler: 
-- Flags: -march=native
-- Build type: Release
-- 
-- The following OPTIONAL packages have been found:
* Threads
-- Configuring done
CMake Warning (dev) at CMakeLists.txt:503 (add_dependencies):
Policy CMP0046 is not set: Error on non-existent dependency in
add_dependencies.  Run "cmake --help-policy CMP0046" for policy details.
Use the cmake_policy command to set the policy and suppress this warning.
The dependency target "clean" of target "distclean" does not exist.
This warning is for project developers.  Use -Wno-dev to suppress it.
-- Generating done
-- Build files have been written to: C:/msys32/home/buildbot/slave/kovri-all-win32/build/deps/cryptopp/build