Allow to specify module specific custom linker flags

Allow to specify module specific custom linker flags
via the MOD_XXX_LDADD variables.

Use APR_ADDTO instead of APR_SETVAR or direct
variable assignment.

This is especially useful when building mod_lua
or mod_deflate against a lua resp. libz which
are installed in non-standard locations.
One can add "-R ..." to MOD_LUA_LDADD and
MOD_DEFLATE_LDADD before configure to fix
the RPATH/RUNPATH of those modules.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1142938 13f79535-47bb-0310-9956-ffa450edef68