Add pkg.m4 upstream

Per https://github.com/php/php-src/pull/6907#issuecomment-826362572.
Closes GH-6907.

[ci skip]

CONTRIBUTING.md

@@ -147,6 +147,7 @@ locations.
     ├─ config.sub                   # https://git.savannah.gnu.org/cgit/config.git
     ├─ libtool.m4                   # https://git.savannah.gnu.org/cgit/libtool.git
     ├─ ltmain.sh                    # https://git.savannah.gnu.org/cgit/libtool.git
+    ├─ pkg.m4                       # https://gitlab.freedesktop.org/pkg-config/pkg-config
     ├─ shtool                       # https://www.gnu.org/software/shtool/
     └─ ...
  ├─ docs/                           # PHP internals and repository documentation