bundled_pjproject: Make it easier to hack

There are times when you need to troubleshoot issues with bundled
pjproject or add new features that need to be pushed upstream
but...

* The source directory created by extracting the pjproject tarball
  is not scanned for code changes so you have to keep forcing
  rebuilds.
* The source directory isn't a git repo so you can't easily create
  patches, do git bisects, etc.
* Accidentally doing a make distclean will ruin your day by wiping
  out the source directory, and your changes.
* etc.

This commit makes that easier.
See third-party/pjproject/README-hacking.md for the details.

ASTERISK-29824

Change-Id: Idb1251040affdab31d27cd272dda68676da9b268

third-party/Makefile.rules

@@ -13,6 +13,9 @@ QUIET_CONFIGURE=
 REALLY_QUIET=
 endif
 
+# The following exports are set during a configure but if
+# it hasn't been run yet, we'll set some defaults
+# to be able to still allow clean and distclean to run
 export SUBMAKE
 export ECHO_PREFIX
 export CMD_PREFIX
@@ -24,12 +27,16 @@ export DESTDIR
 export ASTDATADIR
 export TAR
 export PATCH
-export SED
+export SED ?= sed
 export NM
 export MD5
 export CAT
 export CUT
-export GREP
+export GREP ?= grep
+export FIND ?= find
+export REALPATH ?= realpath
+export BASENAME ?= basename
+export DIRNAME ?= dirname
 export DOWNLOAD
 export DOWNLOAD_TO_STDOUT
 export DOWNLOAD_TIMEOUT