pjproject_bundled: Fix use of LDCONFIG for shared library link creation

pjproject_bundled:  Fix use of LDCONFIG for shared library link creation

LDCONFIG apparently isn't set to something sane on all systems so the creation
of the shared library links fails.  Instead of just testing for non-blank,
main/Makefile now checks that LDCONFIG is actually executable and reverts to
LN if it isn't.

This applies to both libasteriskpj and libasteriskssl.

Thanks to 'abelbeck' for pointing out that the issue was LDCONFIG.

ASTERISK-25873 #close
Reported-by: Hans van Eijsden
Change-Id: I25b76379bc637726ec044b2c0e709b56b3701729