Describe the bug
I ran the following:
gfortran pkg-config --cflags --libs gtk+-3.0 ./gtk-examples/gtkhello.f90 ./gtk-src/gtk.f90

Expected behavior
Expected to compile and run a gui

To Reproduce

Your system:

• OS version: [e.g. Ubuntu 20.04 64 bits]
• Compiler version: [e.g. gfortran 9.3.0]
• GTK branch: [e.g gtk3]

Additional context
Error:

`./gtk-examples/gtkhello.f90:35:6:

35 | use gtk, only: gtk_main_quit, FALSE
| 1
Fatal Error: Cannot open module file ‘gtk.mod’ for reading at (1): No such file or directory
compilation terminated.
./gtk-src/gtk.f90:53:8:

53 | use g, only: g_signal_connect_data
| 1
Fatal Error: Cannot open module file ‘g.mod’ for reading at (1): No such file or directory
compilation terminated.`