cython: use PYTHON_LIBS instead of PYTHON_LDFLAGS

New version of AX_PYTHON_DEVEL macro defines PYTHON_LIBS and not
PYTHON_LDFLAGS, so adjust where needed
diff --git a/cython/Makefile.am b/cython/Makefile.am
index fe4330e..24e9dea 100644
--- a/cython/Makefile.am
+++ b/cython/Makefile.am
@@ -19,7 +19,7 @@
 plist_LTLIBRARIES = plist.la
 plist_la_SOURCES = plist_util.c plist_util.h plist.pyx
 plist_la_CFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/src $(PYTHON_CPPFLAGS) $(AM_CFLAGS) -Wno-shadow -Wno-redundant-decls -Wno-switch-default -Wno-strict-aliasing -Wno-implicit-function-declaration -fvisibility=default
-plist_la_LDFLAGS = -module -avoid-version $(PYTHON_LDFLAGS) $(AM_LDFLAGS)
+plist_la_LDFLAGS = -module -avoid-version $(PYTHON_LIBS) $(AM_LDFLAGS)
 plist_la_LIBADD = $(top_builddir)/src/libplist.la
 
 plist.c: plist.pyx $(PXDINCLUDES) $(PXIINCLUDES)