pjproject: bump to 2.12.1

pjproject: bump to 2.12.1

This is a manual cherry-pick of a57228588602e5217b95f1e9ded0b76402cb6c72
and 472f1ac7ac867b8fbc2841c56e7d61935dba9d52. The only difference is
that the uclibc-related patches were not dropped, because uclibc is used
in OpenWrt 21.02.

- add "--disable-android-mediacodec" to configure
- add EXCLUDE_APP=1 to "make" calls so some apps aren't built (speeds up
  the build a bit)
- drop "sed" call as no longer needed
- update 0004-config_site.patch to sync up with Asterisk 18.14.0
- update 0006-fix-pkg_config-file.patch as there were some changes in
  this area upstream
- add 0007-execinfo.patch to prevent errors due to missing <execinfo.h>
- sync patches with Asterisk 18.14.0
- backports two security fixes (c4d3498 and 450baca) from pjproject
  2.13, source: asterisk 20.0.1

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>