pthsem: fix musl and fortify source compatibility

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>

libs/pthsem/Makefile

 
 PKG_NAME:=pthsem
 PKG_VERSION:=2.0.8
-PKG_RELEASE:=3
+PKG_RELEASE:=4
 
 PKG_MAINTAINER:=Othmar Truniger <github@truniger.ch>
 PKG_LICENSE:=LGPL-2.1+
 
 PKG_FIXUP:=autoreconf
 PKG_BUILD_PARALLEL:=1
+PKG_FORTIFY_SOURCE:=0
 PKG_INSTALL:=1
 
 include $(INCLUDE_DIR)/package.mk
   pthsem is an extend version, with support for semaphores added. It can be installed parallel to a normal pth.
 endef
 
+# The musl libc provides a proper implementation of sigaltstack() so
+# prevent configure from wrongly assuming a broken Linux platform
+ifeq ($(CONFIG_USE_MUSL),y)
+  CONFIGURE_VARS += \
+	ac_cv_check_sjlj=ssjlj
+endif
+
 define Build/InstallDev
 	$(INSTALL_DIR) $(1)/usr/bin
 	$(INSTALL_BIN) $(PKG_INSTALL_DIR)/usr/bin/pthsem-config $(1)/usr/bin/