Add PG_CFLAGS, PG_CXXFLAGS, and PG_LDFLAGS variables to PGXS

Add PG_CFLAGS, PG_CXXFLAGS, and PG_LDFLAGS variables to pgxs.mk which
will be appended or prepended to the corresponding make variables.
Notably, there was previously no way to pass custom CXXFLAGS to third
party extension module builds, COPT and PROFILE supporting only CFLAGS
and LDFLAGS.

Backpatch all the way down to ease integration with existing
extensions.

Author: Christoph Berg
Reviewed-by: Andres Freund, Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/20181113104005.GA32154@msg.credativ.de
Backpatch-through: 9.4

Author: michaelpq

doc/src/sgml/extend.sgml

@@ -1325,7 +1325,34 @@ include $(PGXS)
       <term><varname>PG_CPPFLAGS</varname></term>
       <listitem>
        <para>
-        will be added to <varname>CPPFLAGS</varname>
+        will be prepended to <varname>CPPFLAGS</varname>
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><varname>PG_CFLAGS</varname></term>
+      <listitem>
+       <para>
+        will be appended to <varname>CFLAGS</varname>
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><varname>PG_CXXFLAGS</varname></term>
+      <listitem>
+       <para>
+        will be appended to <varname>CXXFLAGS</varname>
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><varname>PG_LDFLAGS</varname></term>
+      <listitem>
+       <para>
+        will be prepended to <varname>LDFLAGS</varname>
        </para>
       </listitem>
      </varlistentry>

src/makefiles/pgxs.mk

@@ -49,7 +49,10 @@
 #   NO_INSTALLCHECK -- don't define an installcheck target, useful e.g. if
 #     tests require special configuration, or don't use pg_regress
 #   EXTRA_CLEAN -- extra files to remove in 'make clean'
-#   PG_CPPFLAGS -- will be added to CPPFLAGS
+#   PG_CPPFLAGS -- will be prepended to CPPFLAGS
+#   PG_CFLAGS -- will be appended to CFLAGS
+#   PG_CXXFLAGS -- will be appended to CXXFLAGS
+#   PG_LDFLAGS -- will be prepended to LDFLAGS
 #   PG_LIBS -- will be added to PROGRAM link line
 #   PG_LIBS_INTERNAL -- same, for references to libraries within build tree
 #   SHLIB_LINK -- will be added to MODULE_big link line
@@ -116,6 +119,15 @@ endif
 ifdef PG_CPPFLAGS
 override CPPFLAGS := $(PG_CPPFLAGS) $(CPPFLAGS)
 endif
+ifdef PG_CFLAGS
+override CFLAGS := $(CFLAGS) $(PG_CFLAGS)
+endif
+ifdef PG_CXXFLAGS
+override CXXFLAGS := $(CXXFLAGS) $(PG_CXXFLAGS)
+endif
+ifdef PG_LDFLAGS
+override LDFLAGS := $(PG_LDFLAGS) $(LDFLAGS)
+endif
 
 # logic for HEADERS_* stuff