Switch to using explicit WARN_CFLAGS

The rationale is documented well in the automake manual.

Makefile-libostree.am

@@ -31,5 +31,5 @@ libostree_la_SOURCES = libostree/ostree.h \
 	libostree/ostree-checkout.c \
 	libostree/ostree-checkout.h \
 	$(NULL)
-libostree_la_CFLAGS = -I$(srcdir)/libotutil -I$(srcdir)/libostree -DLOCALEDIR=\"$(datadir)/locale\" $(OT_COREBIN_DEP_CFLAGS)
+libostree_la_CFLAGS = $(AM_CFLAGS) -I$(srcdir)/libotutil -I$(srcdir)/libostree -DLOCALEDIR=\"$(datadir)/locale\" $(OT_COREBIN_DEP_CFLAGS)
 libostree_la_LIBADD = libotutil.la $(OT_COREBIN_DEP_LIBS)

Makefile-ostree.am

@@ -34,5 +34,5 @@ ostree_SOURCES = ostree/main.c \
 	ostree/ot-builtin-rev-parse.c \
 	ostree/ot-builtin-show.c \
 	$(NULL)
-ostree_CFLAGS = -I$(srcdir)/libotutil -I$(srcdir)/libostree -I$(srcdir)/ostree  -DLOCALEDIR=\"$(datadir)/locale\" $(OT_COREBIN_DEP_CFLAGS)
+ostree_CFLAGS = $(AM_CFLAGS) -I$(srcdir)/libotutil -I$(srcdir)/libostree -I$(srcdir)/ostree  -DLOCALEDIR=\"$(datadir)/locale\" $(OT_COREBIN_DEP_CFLAGS)
 ostree_LDADD = libotutil.la libostree.la $(OT_COREBIN_DEP_LIBS)

Makefile-otutil.am

@@ -32,5 +32,5 @@ libotutil_la_SOURCES = \
 	libotutil/ot-glib-compat.h \
 	libotutil/otutil.h \
 	$(NULL)
-libotutil_la_CFLAGS = -I$(srcdir)/libotutil -DLOCALEDIR=\"$(datadir)/locale\" $(GIO_UNIX_CFLAGS)
+libotutil_la_CFLAGS = $(AM_CFLAGS) -I$(srcdir)/libotutil -DLOCALEDIR=\"$(datadir)/locale\" $(GIO_UNIX_CFLAGS)
 libotutil_la_LIBADD = $(GIO_UNIX_LIBS)

Makefile.am

@@ -17,6 +17,7 @@
 
 ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
 AM_CPPFLAGS = -DDATADIR='"$(datadir)"' -DLIBEXECDIR='"$(libexecdir)"'
+AM_CFLAGS = $(WARN_CFLAGS)
 
 NULL = 
 BUILT_SOURCES =

configure.ac

@@ -13,16 +13,10 @@ AM_PROG_CC_C_O
 
 changequote(,)dnl
 if test "x$GCC" = "xyes"; then
-  case " $CFLAGS " in
-  *[\ \	]-Wall[\ \	]*) ;;
-  *) CFLAGS="$CFLAGS -Wall" ;;
-  esac
-  case " $CFLAGS " in
-  *[\ \	]-Werror=missing-prototypes[\ \	]*) ;;
-  *) CFLAGS="$CFLAGS -Werror=missing-prototypes" ;;
-  esac
+  WARN_CFLAGS="-Wall -Werror=missing-prototypes"
 fi
 changequote([,])dnl
+AC_SUBST(WARN_CFLAGS)
 
 # Initialize libtool
 AC_PROG_LIBTOOL