[PATCH alternative] 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.
---
NB: This is an alternative approach instead of passing -U_FORTIFY_SOURCE.
I think the -U approach is better, but throwing this up for discussion.

 newlib/libc/include/sys/features.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/newlib/libc/include/sys/features.h b/newlib/libc/include/sys/features.h
index 2188071785f6..6b4999e83482 100644
--- a/newlib/libc/include/sys/features.h
+++ b/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
-- 
2.33.0