cdefs: Limit definition of fortification macros

Define the __glibc_fortify and other macros only when __FORTIFY_LEVEL >
0.  This has the effect of not defining these macros on older C90
compilers that do not have support for variable length argument lists.

Also trim off the trailing backslashes from the definition of
__glibc_fortify and __glibc_fortify_n macros.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>