Move '-D_GNU_SOURCE' to 'configure'

Makefile

@@ -22,7 +22,7 @@ LOBJS = src/nameinfo.o src/version.o \
 
 GENH = src/version.h
 
-CFLAGS  += -Wall -O0 -g -std=c99 -D_GNU_SOURCE -pipe
+CFLAGS  += -Wall -O0 -g -std=c99 -pipe
 NO_AS_NEEDED = -Wl,--no-as-needed
 LIBDL   = -ldl
 LDFLAGS = -fPIC $(NO_AS_NEEDED) $(LIBDL) -lpthread

configure

@@ -142,6 +142,9 @@ if [ -z "$CC" ] ; then
 	CC=cc
 fi
 
+check_compile "whether fine to define _GNU_SOURCE" "-D_GNU_SOURCE" \
+'#include <unistd.h>\nint main() { return 0; }'
+
 for i in 700 600; do
 	check_compile "whether _XOPEN_SOURCE=$i is suitable for C99 mode" "-D_XOPEN_SOURCE=$i" "#include <unistd.h>" && break
 done