Pass explicit linker flags to the Perl bindings build.
This unbreaks 'make check-swig-pl' in cases where the libsvn libraries being tested are not installed in the standard system library search path. With another set of libsvn libraries present on the system, the Perl bindings would get linked to that set, and the tests would fail crashing left and right.
* build/ac-macros/swig.m4 (SVN_FIND_SWIG): Ask Perl which LD flags to use for linking extensions. Expose these flags in a new SWIG_PL_LINK variable.
* subversion/bindings/swig/perl/native/Makefile.PL.in (config): Pass LDFLAGS and LDDLFLAGS explicitly, adding our local libraries to the linker's command line, instead of using whatever defaults the Perl installation happens to use.