libpri: fix build on macos

libpri: fix build on macos

libpri can not be built on macos for OpenWrt Linux target due to:
1. Makefile uses `ar` and `ranlib` (without using make variables).
   MacOS system ar and ranlib are not compatible with the objects
   generated by OpenWrt GCC toolchain. This commit adds patch to
   add an ability to redefine `ar` and `ranlib` tools.
   Upstream issue: https://issues.asterisk.org/jira/browse/PRI-188
2. Makefile detects Darwin using `uname -s` and changes build logic
   but it is not need for cross-platfrom build. This commit
   redefines OSARCH=Linux in OpenWrt Makefile
3. After redefining OSARCH=Linux, libpri Makefile uses /sbin/ldconfig
   that does not exist on MacOS. This commit redefines LDCONFIG=ldconfig
   in OpenWrt Makefile to use ldconfig provided by OpenWrt.

Patch '001-fix-include-signal-h-warning.patch' was refreshed to pass
CI checks. The payload of this patch was not changed.

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>