Accept slightly grotty coding in Makefile.global in order to keep the -L

Accept slightly grotty coding in Makefile.global in order to keep the -L
flag for src/port/ in front of any -L flags placed in LDFLAGS by configure.
This undoes an L-flag-ordering change that I had thought would be safe,
but seems to be making at least one buildfarm member fail --- the only
theory for orca's failure that I can think of is that it's got an old
copy of libpgport.a in /usr/lib.  Also allow for LDFLAGS_SL to be set by
contrib makefiles before they invoke Makefile.global.