Update glib and glibmm to 2.41.2
glib, glibmm: Update to 2.42.0

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
flex, libpng: Use $(SOURCEFORGE_MIRROR)

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
flex: Update

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Merge branch 'update-glib'

[Lluixhi Scura]
- Update glib and glibmm to 2.41.2

[Timothy Gu]
- Update glib and glibmm to 2.42.0

This merges pull request #453.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>