Unix build: Add support for environment variable

EXTRA_LDFLAGS_SHLIB, which allows the user to add to the hard-coded
ld flags specified for DSOs.  Compare with the existing LDFLAGS_SHLIB
environment variable, which allows the user to completely replace the
hard-coded ld flags specified for DSOs.

Reviewed by:	Jim Jagielski, Justin Erenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@98737 13f79535-47bb-0310-9956-ffa450edef68