build: colorize gcc only if on tty

Rather than forcing gcc to always produce colorized error messages
whether on tty or not, enable automatic colorization by ensuring
GCC_COLORS is set to a non-empty string.

Doing it this way removes the need for workarounds in ~/.emacs or
~/.vimrc for "M-x compile" or ":make", respectively, to work.

Makefile.am

@@ -24,6 +24,9 @@ ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
 AM_MAKEFLAGS = --no-print-directory
 AUTOMAKE_OPTIONS = color-tests parallel-tests
 
+GCC_COLORS ?= 'ooh, shiny!'
+export GCC_COLORS
+
 SUBDIRS = . po
 
 # remove targets if the command fails

configure.ac

@@ -190,7 +190,6 @@ CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\
         -ffast-math \
         -fno-common \
         -fdiagnostics-show-option \
-        -fdiagnostics-color \
         -fno-strict-aliasing \
         -fvisibility=hidden \
         -ffunction-sections \