Index: ObjPgSQL.oc.in
==================================================================
--- ObjPgSQL.oc.in
+++ ObjPgSQL.oc.in
@@ -1,5 +1,5 @@
 package_format 1
-CPPFLAGS="$CPPFLAGS @LIBPQ_CPPFLAGS@"
+CPPFLAGS="$CPPFLAGS -I@includedir@ @LIBPQ_CPPFLAGS@"
 LIBS="-L@libdir@ -lobjpgsql @LIBPQ_LIBS@ $LIBS"
 FRAMEWORK_LIBS="-L@libdir@ -lobjpgsql @LIBPQ_LIBS@ $FRAMEWORK_LIBS"
 STATIC_LIBS="@libdir@/libobjpgsql.a @LIBPQ_LIBS@ $STATIC_LIBS"

Index: meson.build
==================================================================
--- meson.build
+++ meson.build
@@ -29,10 +29,11 @@
 
 configure_file(
   input: 'ObjPgSQL.oc.in',
   output: 'ObjPgSQL.oc',
   configuration: {
+    'includedir': get_option('prefix') / get_option('includedir'),
     'libdir': get_option('prefix') / get_option('libdir'),
     'LIBPQ_CPPFLAGS': libpq_cflags,
     'LIBPQ_LIBS': libpq_libs,
   },
   install: true,