libx11.inc: fix build issues for older CentOS distros

libx11.inc: fix build issues for older CentOS distros

Fixes these sorts of issues present on older gcc (CentOS 5.x in this case)

| cc1: error: unrecognized command line option "-Werror=implicit"
| cc1: error: unrecognized command line option "-Werror=nonnull"
| cc1: error: unrecognized command line option "-Werror=init-self"
| cc1: error: unrecognized command line option "-Werror=main"
| cc1: error: unrecognized command line option "-Werror=missing-braces"
| cc1: error: unrecognized command line option "-Werror=sequence-point"
| cc1: error: unrecognized command line option "-Werror=return-type"
| cc1: error: unrecognized command line option "-Werror=trigraphs"
| cc1: error: unrecognized command line option "-Werror=array-bounds"
| cc1: error: unrecognized command line option "-Werror=write-strings"
| cc1: error: unrecognized command line option "-Werror=address"
| cc1: error: unrecognized command line option "-Werror=int-to-pointer-cast"
| cc1: error: unrecognized command line option "-Werror=pointer-to-int-cast"

Also fixes:

makekeys-makekeys.o: In function `main':
makekeys.c:(.text+0x85): undefined reference to `__isoc99_sscanf'
makekeys.c:(.text+0xa7): undefined reference to `__isoc99_sscanf'
collect2: ld returned 1 exit status
make: *** [makekeys] Error 1

Older libc do not have this defined, we can use the -D_GNU_SOURCE
to the compiler to prevent generating calls to this function and
make linking work

(From OE-Core rev: 83c560ae282c1a28fd2c311c66debd02a69f1678)

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>