meson: Add support orc fallback

Allow fallback to orc subproject if any.
Additionally 'dependencies' keyword is removed from find_library,
because it's invalid keyword for find_library.
8 jobs for <span class="ref-name">orc-fallback</span> in 19 minutes and 4 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #93666
gst indent

00:00:34

passed #93665
manifest

00:00:39

 
  Build
passed #93667
build fedora x86_64

00:03:24

 
  Test
passed #93668
check fedora

00:03:17

 
  Full Builds
passed #93669
build android arm64 api28

00:05:27

passed #93671
build cerbero cross win32

00:08:57

passed #93672
build cerbero cross win64

00:07:30

passed #93670
build cerbero fedora x86_64

00:11:42