Bugfix: Undefine _FORTIFY_SOURCE before redefining it, to avoid warnings on compilers that define it by default

configure.ac

@@ -286,7 +286,12 @@ if test x$use_hardening != xno; then
   AX_CHECK_COMPILE_FLAG([-Wstack-protector],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -Wstack-protector"])
   AX_CHECK_COMPILE_FLAG([-fPIE],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fPIE"])
 
-  AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=2"])
+  AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[
+    AX_CHECK_PREPROC_FLAG([-U_FORTIFY_SOURCE],[
+      HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -U_FORTIFY_SOURCE"
+    ])
+    HARDENED_CPPFLAGS="$HARDENED_CPPFLAGS -D_FORTIFY_SOURCE=2"
+  ])
 
   AX_CHECK_LINK_FLAG([[-Wl,--dynamicbase]], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,--dynamicbase"])
   AX_CHECK_LINK_FLAG([[-Wl,--nxcompat]], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -Wl,--nxcompat"])