build: pass -std=gnu99 to compilerlr/compiler-options

With --enable-more-warnings, we already used -std=gnu99, see
NetworkManager commit ba2b2de3adcf3e8286249523e3841858e0c2655c.

Compilation may behave differently depending on the selected
C standard that we choose. It seems wrong, with more-warnings,
to build against a C standard, while otherwise leaving it undefind.

Indeed, one might argue, that our build system should not use
such compiler specific options. At least, not without detecting
support for the compiler option during ./configure.

However:

- we already did this for --enable-more-warnings.

- we should not program against a theoretical compiler. In practice,
  only gcc and clang works to build NetworkManager. Both these compilers
  support this option, so there is no reason to not use it. If we ever
  come into the situation to support another compiler, adjusting -std=gnu99
  will be the smallest problem. Until that happens (and that's far from
  imminent), don't pretend to be portable to non-existing compilers and
  use the flag that in practice is available.

[lkundrak@v3.sk: taken this from NetworkManager, did the Makefile.am
and a trivial commit message adjustment.]

See-also: https://gcc.gnu.org/onlinedocs/gcc/Standards.html

(Taken from NetworkManager commit b9bc20f4da12f15b8950daea177e6dda42cd6c3f)

2 files changed, 13 insertions, 1 deletions

diff --git a/Makefile.am b/Makefile.am
index 3ad1af9a..601e5e46 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -55,6 +55,8 @@ SUBDIRS = \
 	. \
 	po
 
+dflt_cppflags = -std=gnu99
+
 ###############################################################################
 
 icon16dir = $(datadir)/icons/hicolor/16x16/apps
@@ -171,6 +173,7 @@ shared_files = \
 noinst_LTLIBRARIES += src/utils/libutils-libnm.la
 
 src_utils_libutils_libnm_la_CPPFLAGS = \
+	$(dflt_cppflags) \
 	"-I$(srcdir)/shared" \
 	$(GTK3_CFLAGS) \
 	$(LIBNM_CFLAGS)
@@ -189,6 +192,7 @@ if WITH_LIBNM_GTK
 noinst_LTLIBRARIES += src/utils/libutils-libnm-glib.la
 
 src_utils_libutils_libnm_glib_la_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-DNETWORKMANAGER_COMPILATION=NM_NETWORKMANAGER_COMPILATION_LIB_LEGACY \
 	"-I$(srcdir)/shared" \
 	$(GTK3_CFLAGS) \
@@ -210,6 +214,7 @@ check_programs += src/utils/tests/test-utils
 src_utils_tests_test_utils_SOURCES = src/utils/tests/test-utils.c
 
 src_utils_tests_test_utils_CPPFLAGS = \
+	$(dflt_cppflags) \
 	"-I$(srcdir)/shared/" \
 	"-I$(srcdir)/src/utils" \
 	$(GTK3_CFLAGS) \
@@ -280,6 +285,7 @@ nodist_src_wireless_security_libwireless_security_libnm_la_SOURCES = \
 	$(wireless_security_c_gen)
 
 src_wireless_security_libwireless_security_libnm_la_CPPFLAGS = \
+	$(dflt_cppflags) \
 	"-I$(srcdir)/shared" \
 	"-I$(srcdir)/src/utils" \
 	"-I$(srcdir)/src/libnma" \
@@ -305,6 +311,7 @@ nodist_src_wireless_security_libwireless_security_libnm_glib_la_SOURCES = \
 	$(wireless_security_c_gen)
 
 src_wireless_security_libwireless_security_libnm_glib_la_CPPFLAGS = \
+	$(dflt_cppflags) \
 	$(GTK3_CFLAGS) \
 	-DNETWORKMANAGER_COMPILATION=NM_NETWORKMANAGER_COMPILATION_LIB_LEGACY \
 	"-I$(srcdir)/shared" \
@@ -391,6 +398,7 @@ nodist_src_libnm_gtk_libnm_gtk_la_SOURCES = \
 	$(libnm_gtk_c_gen)
 
 src_libnm_gtk_libnm_gtk_la_CFLAGS = \
+	$(dflt_cppflags) \
 	-DNETWORKMANAGER_COMPILATION=NM_NETWORKMANAGER_COMPILATION_LIB_LEGACY \
 	-DICONDIR=\""$(datadir)/icons"\" \
 	-DBINDIR=\""$(bindir)"\" \
@@ -448,6 +456,7 @@ endif
 check_programs += src/libnm-gtk/tests/test-mobile-providers
 
 src_libnm_gtk_tests_test_mobile_providers_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-DTEST_DATA_DIR=\""$(abs_srcdir)/src/libnm-gtk/tests"\" \
 	"-I$(srcdir)/shared" \
 	"-I$(srcdir)/src/libnm-gtk" \
@@ -547,6 +556,7 @@ nodist_src_libnma_libnma_la_SOURCES = \
 	$(libnma_c_gen)
 
 src_libnma_libnma_la_CFLAGS = \
+	$(dflt_cppflags) \
 	-DICONDIR=\""$(datadir)/icons"\" \
 	-DBINDIR=\""$(bindir)"\" \
 	-DSYSCONFDIR=\""$(sysconfdir)"\" \
@@ -710,6 +720,7 @@ nodist_src_connection_editor_nm_connection_editor_SOURCES = \
 	$(connection_editor_c_gen)
 
 src_connection_editor_nm_connection_editor_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-DICONDIR=\""$(datadir)/icons"\" \
 	-DBINDIR=\""$(bindir)"\" \
 	-DSYSCONFDIR=\""$(sysconfdir)"\" \
@@ -835,6 +846,7 @@ nodist_src_nm_applet_SOURCES = \
 	$(nm_applet_c_gen)
 
 src_nm_applet_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-DICONDIR=\""$(datadir)/icons"\" \
 	-DBINDIR=\""$(bindir)"\" \
 	-DSYSCONFDIR=\""$(sysconfdir)"\" \
diff --git a/m4/compiler_options.m4 b/m4/compiler_options.m4
index 6266ffa9..d42501e8 100644
--- a/m4/compiler_options.m4
+++ b/m4/compiler_options.m4
@@ -58,7 +58,7 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 	dnl attach it to the CFLAGS.
 	NM_COMPILER_WARNING([$1], [unknown-warning-option], [])
 
-	CFLAGS="$CFLAGS -Wall -std=gnu99"
+	CFLAGS="$CFLAGS -Wall"
 
 	if test "x$set_more_warnings" = xerror; then
 		CFLAGS_MORE_WARNINGS="$CFLAGS_MORE_WARNINGS -Werror"