Remove repetition in gpos Makefile

In a forthcoming commit we're gonna tweak some of these flags. To
prevent duplication, consolidate them through Makefile inclusion first.
This also addresses a FIXME left from commit 3f3f4a57 "Fix configure
and cmake to build ORCA with debug".

src/backend/gporca/libgpos/src/common/Makefile

@@ -8,13 +8,7 @@ subdir = src/backend/gporca/libgpos/src/common
 top_builddir = ../../../../../..
 include $(top_builddir)/src/Makefile.global
 
-override CPPFLAGS := -I$(top_builddir)/src/backend/gporca/libgpos/include $(CPPFLAGS)
-override CPPFLAGS := -I$(top_builddir)/src/backend/gporca/libgpopt/include $(CPPFLAGS)
-override CPPFLAGS := -I$(top_builddir)/src/backend/gporca/libnaucrates/include $(CPPFLAGS)
-override CPPFLAGS := -I$(top_builddir)/src/backend/gporca/libgpdbcost/include $(CPPFLAGS)
-# FIXME: Would be better to include gporca.mk, but hitting a warning
-override CPPFLAGS := -Wno-variadic-macros -fno-omit-frame-pointer $(CPPFLAGS)
-override CPPFLAGS := -std=gnu++98 $(CPPFLAGS)
+include $(top_builddir)/src/backend/gporca/gporca.mk
 
 OBJS        = CAutoTimer.o \
               CBitSet.o \