Makefile: fix _FORTIFY_SOURCE redefined build error

Certain builds of gcc enable _FORTIFY_SOURCE which results in the error:
<command-line>:0:0: warning: "_FORTIFY_SOURCE" redefined
<command-line>:0:0: note: this is the location of the previous definition

Correct this by undefining it first and redefining it. Also, the previous
command line option was using -Wp which is passing the value *AS IS* to the
pre-processor rather than to the compiler driver. The C pre-processor has
an undocumented interface subject to change per man 1 gcc. Just use the
-D option to specify this value.

Signed-off-by: William Roberts <william.c.roberts@intel.com>

libselinux/src/Makefile

@@ -64,7 +64,7 @@ ifeq ($(COMPILER), gcc)
 EXTRA_CFLAGS = -fipa-pure-const -Wlogical-op -Wpacked-bitfield-compat -Wsync-nand \
 	-Wcoverage-mismatch -Wcpp -Wformat-contains-nul -Wnormalized=nfc -Wsuggest-attribute=const \
 	-Wsuggest-attribute=noreturn -Wsuggest-attribute=pure -Wtrampolines -Wjump-misses-init \
-	-Wno-suggest-attribute=pure -Wno-suggest-attribute=const -Wp,-D_FORTIFY_SOURCE
+	-Wno-suggest-attribute=pure -Wno-suggest-attribute=const -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE
 else
 EXTRA_CFLAGS = -Wunused-command-line-argument
 endif

libselinux/utils/Makefile

@@ -30,7 +30,7 @@ CFLAGS ?= -O -Wall -W -Wundef -Wformat-y2k -Wformat-security -Winit-self -Wmissi
           -Wformat-extra-args -Wformat-zero-length -Wformat=2 -Wmultichar \
           -Woverflow -Wpointer-to-int-cast -Wpragmas \
           -Wno-missing-field-initializers -Wno-sign-compare \
-          -Wno-format-nonliteral -Wframe-larger-than=$(MAX_STACK_SIZE) -Wp,-D_FORTIFY_SOURCE \
+          -Wno-format-nonliteral -Wframe-larger-than=$(MAX_STACK_SIZE) -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE \
           -fstack-protector-all --param=ssp-buffer-size=4 -fexceptions \
           -fasynchronous-unwind-tables -fdiagnostics-show-option -funit-at-a-time \
           -Werror -Wno-aggregate-return -Wno-redundant-decls \