autoconf: Only _GNU_SOURCE feature test macro needed on gnu systems

According to feature_test_macros(7), _GNU_SOURCE encompasses all the
other feature macros we were setting, so we can just dispose of them.

configure.ac

@@ -84,10 +84,7 @@ DEFINES=""
 AC_SUBST([DEFINES])
 case "$host_os" in
 *-gnu*)
-if test "x$GCC" = xyes; then
-    DEFINES="$DEFINES -D_POSIX_SOURCE -D_POSIX_C_SOURCE=199309L -D_BSD_SOURCE"
-fi
-    DEFINES="$DEFINES -D_SVID_SOURCE -D_GNU_SOURCE -DPTHREADS"
+    DEFINES="$DEFINES -D_GNU_SOURCE -DPTHREADS"
     ;;
 solaris*)
     DEFINES="$DEFINES -DPTHREADS -DSVR4"