Avoid using flags not supported by in-tree gcc.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -20,14 +20,15 @@ mark_as_advanced(CMAKE_INSTALL_PREFIX)
 option(USE_WIDECHAR "Enable wide character support" ON)
 option(USE_ICONV "Enable iconv support" ON)
 
-add_compile_options(-fcolor-diagnostics)
 add_compile_options($<$<CONFIG:Debug>:-Wall>)
 add_compile_options($<$<CONFIG:Debug>:-Wno-parentheses>)
 add_compile_options($<$<CONFIG:Debug>:-Wno-uninitialized>)
 add_compile_options($<$<CONFIG:Debug>:-Wmissing-prototypes>)
 add_compile_options($<$<CONFIG:Debug>:-Wsystem-headers>)
 add_compile_options($<$<CONFIG:Release>:-Wuninitialized>)
-add_compile_options($<$<CONFIG:Release>:-Wno-dangling-else>)
+if ("${CMAKE_C_COMPILER_ID}" STREQUAL "Clang")
+    add_compile_options($<$<CONFIG:Release>:-Wno-dangling-else>)
+endif()
 add_compile_options(-Wstack-protector -fstack-protector)
 add_compile_options(-Wstrict-aliasing -fstrict-aliasing)