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>
