ecpg: Don't link compatlib with libpq

It doesn't actually use libpq.  But we need to keep libpq in the
CPPFLAGS for building, because compatlib uses ecpglib.h which uses
libpq-fe.h, but we don't need to refer to libpq for linking.

reviewed by Tom Lane

src/interfaces/ecpg/compatlib/Makefile

@@ -21,7 +21,7 @@ override CPPFLAGS := -I../include -I$(top_srcdir)/src/interfaces/ecpg/include \
 	-I$(libpq_srcdir) -I$(top_srcdir)/src/include/utils $(CPPFLAGS)
 override CFLAGS += $(PTHREAD_CFLAGS)
 
-SHLIB_LINK = -L../ecpglib -lecpg -L../pgtypeslib -lpgtypes $(libpq) \
+SHLIB_LINK = -L../ecpglib -lecpg -L../pgtypeslib -lpgtypes \
 	$(filter -lintl -lm, $(LIBS)) $(PTHREAD_LIBS)
 SHLIB_PREREQS = submake-ecpglib submake-pgtypeslib