Build libecpg with -DFRONTEND in all supported versions.

Fix an oversight in commit 151e747 by
back-patching commit 44c5d38 to 9.0.

Author: nmisch

src/interfaces/ecpg/ecpglib/Makefile

@@ -18,7 +18,7 @@ SO_MAJOR_VERSION= 6
 SO_MINOR_VERSION= 5
 
 override CPPFLAGS := -I../include -I$(top_srcdir)/src/interfaces/ecpg/include \
-	-I$(libpq_srcdir) -I$(top_builddir)/src/port $(CPPFLAGS)
+	-I$(libpq_srcdir) -I$(top_builddir)/src/port -DFRONTEND $(CPPFLAGS)
 override CFLAGS += $(PTHREAD_CFLAGS)
 
 # Need to recompile any libpgport object files