[libcxxabi] PR26778 Fix tests when CXX="ccache clang++"

[hintonda]

Summary

If you use ccache to build libcxxabi by setting CXX="ccache
clang++", check-libcxxabi will fail since LIBCXXABI_COMPILER will be
set to "ccache".

We can't just include CMAKE_CXX_COMPILER_ARG1 because of a python
parsing issue, so the best we can do right now is to use
CMAKE_CXX_COMPILER_ARG1 if it exists, and CMAKE_CXX_COMPILER if it
doesn't.

[hintonda]

If *_COMPILER_ARG1 is set, then it shall be the compiler, and the value for CMAKE_<LANG>_COMPILER may be ccache, but could also be something else, e.g., distcc, etc. Therefore, I don't see a clean way to check it.

However, newer versions of cmake have abandoned the *_COMPILER_ARG1 solutions and set CMAKE_<LANG>_COMPILER to the real compiler and use a new variable, CMAKE_<LANG>_COMPILER_LAUNCHER for ccache, distcc, etc..

So the solution is just to upgrade to the newest version of cmake if you want to use ccache, etc...

CMakeLists.txt

set(CMAKE_MODULE_PATH
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules"
   ${CMAKE_MODULE_PATH}
   )
 
 # Configure compiler.
 include(config-ix)
 
-set(LIBCXXABI_COMPILER    ${CMAKE_CXX_COMPILER})
+# When ccache is used, the actual compiler will be in arg1.  Solves
+# a python parsing issue in CXXCompiler() where parameters with spaces are
+# treated as seperate parameters.
+if(CMAKE_CXX_COMPILER_ARG1)
+  string(STRIP ${CMAKE_CXX_COMPILER_ARG1} LIBCXXABI_COMPILER )
+else()
+  set(LIBCXXABI_COMPILER ${CMAKE_CXX_COMPILER})
+endif()
 set(LIBCXXABI_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBCXXABI_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
 set(LIBCXXABI_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXXABI_LIBDIR_SUFFIX})
 
 #===============================================================================
 # Setup Compiler Flags
 #===============================================================================