contrib: use a more specific name for the build folder

It should be safe to delete build folders in one call: find . -mindepth 3 -maxdepth 4 -type d -name vlc_build -exec rm -rf {} ;

Plus a fix contrib fixes/cleanup.

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v2:

• add BUILD_DIRUNPACK since pkg_static works inside UNPACK_DIR
• avoid patching .pc files during build

v3:

• remove fontconfig changes, it's built out of tree yet

v4:

• Draft on top of !2601 (closed)
• fix BUILD_DIRUNPACK bisecting