Index: ObjMatrix.oc.in
==================================================================
--- ObjMatrix.oc.in
+++ ObjMatrix.oc.in
@@ -1,6 +1,7 @@
 package_format 1
 package_depends_on ObjFWTLS
 package_depends_on ObjSQLite3
+CPPFLAGS="$CPPFLAGS -I@includedir@"
 LIBS="-L@libdir@ -lobjmatrix $LIBS"
 FRAMEWORK_LIBS="-L@libdir@ -lobjmatrix $FRAMEWORK_LIBS"
 STATIC_LIBS="@libdir@/libobjmatrix.a $STATIC_LIBS"

Index: meson.build
==================================================================
--- meson.build
+++ meson.build
@@ -20,9 +20,10 @@
 
 configure_file(
   input: 'ObjMatrix.oc.in',
   output: 'ObjMatrix.oc',
   configuration: {
+    'includedir': get_option('prefix') / get_option('includedir'),
     'libdir': get_option('prefix') / get_option('libdir'),
   },
   install: true,
   install_dir: packages_dir)