See https://trac.macports.org/ticket/61910
V currently only supports dynamically setting C Flags from environment variables.

--- vlib/db/pg/pg.c.v.original	2023-02-07 11:37:41.000000000 +0000
+++ vlib/db/pg/pg.c.v	2023-02-07 11:38:58.000000000 +0000
@@ -9,17 +9,8 @@
 	#flag -lpq
 	#flag linux -I/usr/include/postgresql
 
-	#flag darwin -I/opt/local/include/postgresql11
-	#flag darwin -L/opt/local/lib/postgresql11
-
-	#flag darwin -I/usr/local/opt/libpq/include
-	#flag darwin -L/usr/local/opt/libpq/lib
-
-	#flag darwin -I/opt/homebrew/include
-	#flag darwin -L/opt/homebrew/lib
-
-	#flag darwin -I/opt/homebrew/opt/libpq/include
-	#flag darwin -L/opt/homebrew/opt/libpq/lib
+	#flag darwin -I@PREFIX@/include/postgresql$env('POSTGRESQL_VERSION')
+   #flag darwin -L@PREFIX@/lib/postgresql$env('POSTGRESQL_VERSION')
 
 	#flag windows -I @VEXEROOT/thirdparty/pg/include
 	#flag windows -L @VEXEROOT/thirdparty/pg/win64