string: fix build failure on BSD/OSX with FORTIFY_SOURCE
This avoids a conflict with "FORTIFY_SOURCE" variants
of the string functions when they're replaced on NetBSD-6.0.1
and Darwin-14.3.0 at least. On these platforms, the system
<string.h> re-includes "string.h" through <strings.h>, before
going on to redefine the "FORTIFY_SOURCE" _chk variants.
For example rpl_strncat is redefined to __builtin___strncat_chk.
Note strncat is being replaced as the configure check is failing
with GCC 4.5.0, due to the builtin strncat failing the check.
I.E. when compiled with -O2 the test fails, but passes when
-fno-builtin-strncat is also specified. -D_FORTIFY_SOURCE has
no impact on the configure test result.
* lib/string.in.h: Avoid including our "lib/string.h" while
including the system <string.h>.