Use _DEFAULT_SOURCE instead of _GNU_SOURCE.

--- readpe-0.82.orig/lib/libpe/Makefile
+++ readpe-0.82/lib/libpe/Makefile
@@ -87,7 +87,7 @@
 # may not be loaded.
 #
 
-libpe: CPPFLAGS += -D_GNU_SOURCE
+libpe: CPPFLAGS += -D_DEFAULT_SOURCE
 ifeq ($(PLATFORM_OS), CYGWIN)
 libpe: CPPFLAGS += -D_XOPEN_SOURCE=600
 endif