build: Set default compiler variables from configure

This centralizes the setting so there's no duplication anymore,
makes sure the user supplied variables are never overridden, and
are only set when using gcc.

Reported-by: Samuli Suominen <ssuominen@gentoo.org>

configure.ac

@@ -11,6 +11,9 @@ AM_INIT_AUTOMAKE([1.8 foreign nostdinc])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])],
                             [AC_SUBST([AM_DEFAULT_VERBOSITY], [1])])
 
+# Check and store if we got user supplied variables
+user_CFLAGS=${CFLAGS-unset}
+
 # Checks for operating system services and capabilities.
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
@@ -22,6 +25,11 @@ AC_PROG_CC
 AC_PROG_INSTALL
 AC_PROG_LN_S
 
+# Set default compiler variables
+if test "$user_CFLAGS" = unset && test "$GCC" = yes; then
+  CFLAGS="$CFLAGS -Wall -Wextra -Wno-unused-variable -Wno-unused-parameter"
+fi
+
 # Checks for libraries.
 
 # Checks for header files.

src/Makefile.am

@@ -1,8 +1,5 @@
 ## Process this file with automake to produce Makefile.in
 
-# Set default values for user variables
-CFLAGS = -Wall -Wextra -Wno-unused-variable -Wno-unused-parameter
-
 AM_CPPFLAGS = \
 	-I$(top_builddir) \
 	-isystem $(top_srcdir)/include/bsd/ \

test/Makefile.am

@@ -1,8 +1,5 @@
 ## Process this file with automake to produce Makefile.in
 
-# Set default values for user variables
-CFLAGS = -Wall -Wextra -Wno-unused-variable -Wno-unused-parameter
-
 AM_CPPFLAGS = \
 	-I$(top_builddir) \
 	-isystem $(top_srcdir)/include/bsd/ \