build: Do not force diagnostics-color flag

The -fdiagnostics-color flag is only available on GCC >= 4.9, for
older versions this could raise an error in certain circumstances
(such as when using ccache).  Instead, since -fdiagnostic-color=auto
by default  in gcc-4.9, simply set the required environment variable
to the default one if it's undefined.

Based mostly on the systemd commit f44541bc by Michal Schmidt.

Makefile.am

@@ -11,6 +11,9 @@ BUILT_FILES =
 ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
 AM_MAKEFLAGS = --no-print-directory
 
+GCC_COLORS ?= 'yes'
+export GCC_COLORS
+
 AM_CPPFLAGS = \
 	-include $(top_builddir)/config.h \
 	-I$(top_srcdir)/libkmod \

configure.ac

@@ -201,7 +201,6 @@ CC_CHECK_FLAGS_APPEND(with_cflags, [CFLAGS], [\
 		       -Wuninitialized \
 		       -fno-common \
 		       -fdiagnostics-show-option \
-		       -fdiagnostics-color=auto \
 		       -fvisibility=hidden \
 		       -ffunction-sections \
 		       -fdata-sections])