CMake: Default color diagnostics to force-on with ccache/ninja

Reading diagnostics with ANSI color codes in between is unnecessarily
hard. We now make use of the cmake documented switch to control color
output. And we only force color on if the tooling suggest it might be
necessary.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

CMakeLists.txt

@@ -48,6 +48,14 @@ else ()
     message(STATUS "ccache will not be used")
 endif ()
 
+# Prefer forced colored compiler output if there's a chance we'd otherwise get none at all.
+# Ninja and ccache "consume" the compiler output, breaking the terminal detection of compilers.
+# ccache tries to solve the problem, but can only do so if it determines that it's calling GCC or Clang. It uses a very lightweight heuristic, which breaks easily.
+if ((CMAKE_GENERATOR STREQUAL "Ninja" OR (CCACHE_PROGRAM AND USE_CCACHE)) AND NOT DEFINED CMAKE_COLOR_DIAGNOSTICS)
+    message(STATUS "Forcing compiler color output due to the use of Ninja and/or ccache. Use -DCMAKE_COLOR_DIAGNOSTICS=OFF to turn it off.")
+    set(CMAKE_COLOR_DIAGNOSTICS ON)
+endif()
+
 set(CMAKE_EXT_DEP_WARNING_GUARD "")
 if (DISABLE_EXTERNAL_DEPS_WARNINGS) # enable warnings for external dependencies
     set(CMAKE_EXT_DEP_WARNING_GUARD SYSTEM)
@@ -100,15 +108,14 @@ endif ()
 string(REPLACE ";" " " ALL_COMPILER_FLAGS "${ALL_COMPILER_FLAGS}")
 
 if (CMAKE_CXX_COMPILER_ID MATCHES ".*Clang") # set default C++ STL to Clang's libc++ when using Clang
-    add_compile_options(-stdlib=libc++ -fcolor-diagnostics)
+    add_compile_options(-stdlib=libc++)
     if (TIMETRACE)
         add_compile_options(-ftime-trace)
         set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -ftime-trace")
         message(STATUS "Enable TIMETRACE: ${TIMETRACE}")
     endif ()
     set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -stdlib=libc++ -lc++")
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-    add_compile_options(-fdiagnostics-color=always)
     if (ENABLE_TBB)
         find_package(TBB REQUIRED)
         target_link_libraries(gnuradio-options INTERFACE TBB::tbb)