build: fix warnings on glibc 2.19

glibc 2.19 complains about deprecation of both _GNU_SOURCE and _SVID_SOURCE
defined. (_SVID_SOURCE comes from _BSD_SOURCE). This does a check for
glibc and only defines _GNU_SOURCE if we are on glibc. _GNU_SOURCE covers
all our bases from _BSD_SOURCE anyway.

Author: Christian Hergert

build/autotools/CheckCompiler.m4

@@ -30,6 +30,18 @@ if test "$c_compiler" = "unknown"; then
     AC_MSG_ERROR([Compiler GCC >= 4.1 or Clang >= 3.3 is required for C compilation])
 fi
 
+# GLibc 2.19 complains about both _BSD_SOURCE and _GNU_SOURCE. The _GNU_SOURCE
+# contains everything anyway. So just use that.
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#include <features.h>
+#ifndef __GLIBC__
+#error not glibc
+#endif
+]], [])],
+LIBC_FEATURES="-D_GNU_SOURCE",
+LIBC_FEATURES="-D_BSD_SOURCE")
+AC_SUBST(LIBC_FEATURES)
+
 AC_C_CONST
 AC_C_INLINE
 AC_C_TYPEOF

src/Makefile.am

@@ -9,8 +9,7 @@ lib_LTLIBRARIES += libmongoc-1.0.la
 
 libmongoc_priv_la_CPPFLAGS = \
 	-DMONGOC_COMPILATION \
-	-D_BSD_SOURCE \
-	-D_GNU_SOURCE \
+	$(LIBC_FEATURES) \
 	$(OPTIMIZE_CFLAGS) \
 	-I$(top_srcdir)/src \
 	-I$(top_srcdir)/src/mongoc \