This commit addresses two issues encountered while building pthsem for
x86_64/musl:

1) Autoconf incorrectly assumes a broken Glibc and thus falls back
   to Linux compatibility code which runs into an unsupported
   platform error with musl libc. Since musl provides a proper
   implementation of `sigaltstack(2)`, force configure to use the
   POSIX compliant code when musl libc is used.

2) The `pth_syscall.c` file undefines various libc function names
   which breaks the indirection set up by fortify-source headers,
   therefore forcibly disable fortify source in the OpenWrt Makefile.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>