build: pass -std=gnu99 to compiler

With --enable-more-warnings, we already used -std=gnu99, see
commit ba2b2de3ad.

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.

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

Makefile.am

@@ -105,6 +105,8 @@ endif
 
 endif
 
+dflt_cppflags = -std=gnu99
+
 ###############################################################################
 
 # with autotools, it is not easily possible to generate these defines
@@ -592,6 +594,7 @@ nodist_libnminclude_HEADERS += \
 ###############################################################################
 
 dflt_cppflags_libnm_core = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -1216,6 +1219,7 @@ libnm_tests_libnm_vpn_plugin_utils_test_la_SOURCES = \
 	shared/nm-utils/nm-vpn-plugin-utils.h
 
 libnm_tests_libnm_vpn_plugin_utils_test_la_CFLAGS = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -1322,7 +1326,7 @@ EXTRA_DIST += shared/c-list/src/c-list.h
 ###############################################################################
 
 src_libsystemd_nm_la_cppflags = \
-	$(dflt_cppflags_std) \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -1859,6 +1863,7 @@ EXTRA_DIST += \
 libexec_PROGRAMS += src/dhcp/nm-dhcp-helper
 
 src_dhcp_nm_dhcp_helper_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-DG_LOG_DOMAIN=\""nm-dhcp-helper"\" \
@@ -1920,6 +1925,7 @@ core_plugins += src/ppp/libnm-ppp-plugin.la
 pppd_plugin_LTLIBRARIES += src/ppp/nm-pppd-plugin.la
 
 src_ppp_nm_pppd_plugin_la_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -3152,6 +3158,7 @@ noinst_LTLIBRARIES += \
 	dispatcher/libnm-dispatcher-core.la
 
 dispatcher_cppflags = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -3249,6 +3256,7 @@ EXTRA_DIST += \
 check_programs += dispatcher/tests/test-dispatcher-envp
 
 dispatcher_tests_test_dispatcher_envp_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -3289,6 +3297,7 @@ EXTRA_DIST += \
 bin_PROGRAMS += clients/nm-online
 
 clients_nm_online_CPPFLAGS = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -3317,6 +3326,7 @@ EXTRA_DIST += \
 ###############################################################################
 
 clients_cppflags = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-core \
@@ -3830,6 +3840,7 @@ lib_LTLIBRARIES += libnm-util/libnm-util.la
 libnm_util_libnm_util_includedir=$(includedir)/NetworkManager
 
 libnm_util_cppflags = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/libnm-util \
 	-I$(builddir)/libnm-util \
 	-I$(srcdir)/shared \
@@ -4041,6 +4052,7 @@ dist_dependencies += libnm-util/libnm-util.la
 ###############################################################################
 
 libnm_util_tests_cppflags = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-util \
@@ -4120,6 +4132,7 @@ EXTRA_DIST += \
 ###############################################################################
 
 libnm_glib_cppflags = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-util \
@@ -4398,6 +4411,7 @@ dist_dependencies += \
 ###############################################################################
 
 libnm_glib_tests_cppflags = \
+	$(dflt_cppflags) \
 	-I$(srcdir)/shared \
 	-I$(builddir)/shared \
 	-I$(srcdir)/libnm-util \

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_MORE_WARNINGS="-Wall -std=gnu99"
+	CFLAGS_MORE_WARNINGS="-Wall"
 
 	if test "x$set_more_warnings" = xerror; then
 		CFLAGS_MORE_WARNINGS="$CFLAGS_MORE_WARNINGS -Werror"