newlib: ignore _FORTIFY_SOURCE when building newlib

Some distros enable _FORTIFY_SOURCE by default which upsets building
newlib which itself implements the logic for this define.  For example,
building gets.c fails because the includes set up a gets() macro which
expands in the definition.

Since newlib isn't prepared to build itself with _FORTIFY_SOURCE, and
it's not clear if it's even useful, ignore it when building the code.
This also matches what glibc is doing.

newlib/libc/include/sys/features.h

@@ -320,7 +320,8 @@ extern "C" {
 #endif
 
 #if _FORTIFY_SOURCE > 0 && !defined(__cplusplus) && !defined(__lint__) && \
-   (__OPTIMIZE__ > 0 || defined(__clang__)) && __GNUC_PREREQ__(4, 1)
+   (__OPTIMIZE__ > 0 || defined(__clang__)) && __GNUC_PREREQ__(4, 1) && \
+   !defined(_COMPILING_NEWLIB)
 #  if _FORTIFY_SOURCE > 1
 #    define __SSP_FORTIFY_LEVEL 2
 #  else