build: bump C standard to (gcc's) C11

We already import systemd code which is C11. To get this even
to build, we need workaround like patching import of <uchar.h>.

Also, the libraries from c-util and nettools are C11. We cannot even
compile them in C99 mode (and didn't do that either).

It's time to bump the version. We need C11 from now on (or better: gcc's
dialect of it).

Also, note that since nettools/nacd is not optional, we could not even
build NetworkManager without a C11 compiler. So, just use it everywhere.

Makefile.am

@@ -105,7 +105,7 @@ endif
 
 endif
 
-dflt_cppflags = -std=gnu99
+dflt_cppflags = -std=gnu11
 
 ###############################################################################
 
@@ -222,7 +222,10 @@ shared_libcsiphash_la_SOURCES = \
 
 noinst_LTLIBRARIES += shared/libcrbtree.la
 
-shared_libcrbtree_la_CFLAGS = $(AM_CFLAGS) -std=c11
+shared_libcrbtree_la_CFLAGS = \
+	$(AM_CFLAGS) \
+	-std=c11 \
+	$(NULL)
 
 shared_libcrbtree_la_CPPFLAGS = \
 	$(CODE_COVERAGE_CFLAGS) \