Merge pull request #53 from j-b-m/master

Fix gcc detection
Merge pull request #54 from j-b-m/master

Fix kdenlivetitle clip wrong dimensions
Drop support for FFmpeg < v1.0 and Libav < v9.
Fix AVCodecContext.request_channels deprecation.
Use av_frame_alloc() instead of deprecated avcodec_alloc_frame().
A minor code cleanup in avformat producer.
Prevent starting/stopping a thread on Windows to serialize XML.
Add aligned version of Windows malloc() and free().
Add out-of-memory handling to mlt_pool.
Remove an unnecessary null check in mlt_pool. (SF-216)
Ignore MLT_REPOSITORY env var on Windows and RELOCATABLE Darwin builds.
Revert 279ebd to prevent integer overflow for duration calculation.

The integer math could easily overflow with large number for profile-
auto-profile. To address the Maksym's concern, rounding was added.
Fix regression in commit c50864e on downmix multi-channel audio.

AVCodecContex->channels can change after decoding.
Fix crash using RGB filter with region transition. (kdenlive-3377)
Fix dual pass encoding with libvpx. (github-57)
Add first pass WebM preset. (github-57)