Recognize Clang to be a GCC-compatible compiler

Clang aims to be compatible with GCC when it comes to command-line
parameters. Where we enable functionality based on the presence of
a GCC-compiler, we can use the same functionality with Clang.

This patch introduces a new variable CXX_COMPAT_GCC which is true if
the compiler handles the same options as GCC, and this variable is
subsequently used in tests instead of CMAKE_COMPILER_IS_GNUCXX (which
remains if we need to test if we really have GCC, e.g. for version)

cmake/Modules/UseOptimization.cmake

@@ -2,6 +2,8 @@
 
 include(TestCXXAcceptsFlag)
 include (AddOptions)
+include (UseCompVer)
+is_compiler_gcc_compatible ()
 
 # mapping from profile name (in CMAKE_BUILD_TYPE) to variable part
 set (_prof_DEBUG "Debug")
@@ -11,7 +13,7 @@ set (_prof_RELEASE "Release;RelWithDebInfo;MinSizeRel")
 # otherwise, turn them on. indicate to the code what we have done
 # so it can turn on assertions etc.
 
-if (CMAKE_COMPILER_IS_GNUCXX)
+if (CXX_COMPAT_GCC)
   # extra flags passed for optimization
   set (_opt_flags "")
 
@@ -44,7 +46,7 @@ if (CMAKE_COMPILER_IS_GNUCXX)
   # use these options for release builds - full optimization
   add_options (ALL_LANGUAGES "${_prof_RELEASE}" ${_opt_rel} "-DNDEBUG" ${_opt_flags})
 
-else (CMAKE_COMPILER_IS_GNUCXX)
+else ()
   # default information from system
   foreach (lang IN ITEMS C CXX Fortran)
 	if (lang STREQUAL "Fortran")
@@ -59,4 +61,4 @@ else (CMAKE_COMPILER_IS_GNUCXX)
 	  endif (NOT CMAKE_${lang}_FLAGS_${profile})
 	endforeach (profile)
   endforeach (lang)
-endif (CMAKE_COMPILER_IS_GNUCXX)
+endif ()