[configure] Don't stop build if runtime-dependencies are not available
@bilboed
Submitted by Edward Hervey Assigned to Edward Hervey @bilboed
Description
Currently configure.ac is checking for a lot of dependencies which are not build dependencies but runtime-dependencies.
We should no longer raise errors when configure doesn't find some dependencies at build time, but only warning.
All runtime-dependencies should be checked for in pitivi/check.py instead.
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=406341