FindGTK2: Build tests in separate directories

This should fix the tests failing when running in parallel
FindGTK2: Add libraries to the GTK2_LIBRARIES variable only when found

Some libraries (e.g. gio) are not necessary, and often not available
with older GTK2 versions, therefore GTK_LIBRARIES should not contain
GTK2_XXX-NOT_FOUND for these libraries.
FindGTK2: Do not add freetype includes if they are not found

On some older system they are not required
Merge topic 'FindGTK2-targets' into next

9ed99f7 FindGTK2: Do not add freetype includes if they are not found
47655c4 FindGTK2: Add libraries to the GTK2_LIBRARIES variable only when found
1e7d2b8 FindGTK2: Build tests in separate directories
Revert "FindGTK2: Fail if freetype include directories are not found"

This reverts commit 7de465b7ef3aa61846d9c949357373058820614d.
Merge topic 'FindGTK2-targets' into next

b22e337 Revert "FindGTK2: Fail if freetype include directories are not found"