Fix PL/Perl build on Cygwin

This was broken by b4e9368.  The
reason why this fixes it are not entirely clear, but it seemed the
best way to get it working again.

Discussion: https://www.postgresql.org/message-id/flat/8c4fcb72-2574-ff7c-4c25-1f032d4a2a57%40enterprisedb.com

Author: petere

src/pl/plperl/GNUmakefile

@@ -55,6 +55,11 @@ endif # win32
 
 SHLIB_LINK = $(perl_embed_ldflags)
 
+# see https://www.postgresql.org/message-id/flat/8c4fcb72-2574-ff7c-4c25-1f032d4a2a57%40enterprisedb.com
+ifeq ($(PORTNAME), cygwin)
+SHLIB_LINK += -Wl,--export-all-symbols
+endif
+
 REGRESS_OPTS = --dbname=$(PL_TESTDB)
 REGRESS = plperl_setup plperl plperl_lc plperl_trigger plperl_shared \
 	plperl_elog plperl_util plperl_init plperlu plperl_array \