build-sys: Set C language standard to gnu11

Per glibc feature_test_macros(7), setting compiler flags to
-std=c11 (or any c* variant like c99) enforces strict ANSI
mode.

Enforcing strict ANSI makes all declarations under _GNU_SOURCE
unavailable. This leads to build warnings in the form of:

    warning: implicit declaration of function ‘syscall’

Thus replace -std=c11 with -std=gnu11

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>

configure.ac

@@ -176,9 +176,9 @@ esac
 
 #### Compiler flags ####
 
-AX_CHECK_COMPILE_FLAG([-std=c11],
+AX_CHECK_COMPILE_FLAG([-std=gnu11],
    [],
-   [AC_MSG_ERROR([*** Compiler does not support -std=c11])],
+   [AC_MSG_ERROR([*** Compiler does not support -std=gnu11])],
    [-pedantic -Werror])
 
 AX_APPEND_COMPILE_FLAGS(

src/Makefile.am

@@ -48,7 +48,7 @@ AM_CPPFLAGS = \
 	-DPA_SRCDIR=\"$(abs_srcdir)\" \
 	-DPA_BUILDDIR=\"$(abs_builddir)\" \
 	-DPULSE_LOCALEDIR=\"$(localedir)\"
-AM_CFLAGS = -std=c11 \
+AM_CFLAGS = -std=gnu11 \
 	$(PTHREAD_CFLAGS)
 AM_CXXFLAGS = -std=c++11 \
 	$(PTHREAD_CFLAGS)