Fix the build with the upcoming CMake 3.4.0.

It is not clear if this is intentional or not, but with CMake 3.4.0
environment variables such as CXXFLAGS are passed to the compiler after the
directories added via the include_directories() command. In practice, this
means that we then end up with the following command-line:

${CXX} ... -I/usr/local/include ...
           -I/usr/local/include/ffmpeg0/libavcodec
           -I/usr/local/include/ffmpeg0 ...

which fails because of the following chain of events:

• The port brings in multimedia/ffmpeg indirectly.
• The source code includes headers as <avformat.h> like that, which turn into /usr/local/include/ffmpeg0/libavformat/avformat.h to the compiler.
• Headers like avformat.h itself include other headers with "libavcodec/avcodec.h", which, given the compiler command-line above, becomes /usr/local/include/libavcodec/avcodec.h, coming from multimedia/ffmpeg, not multimedia/ffmpeg0.

Fix it by making the port behave as it should have from the beginning:

• Stop setting C{PP,XX}FLAGS in the Makefile, as it does not help.
• Include the ffmpeg-related headers before the others, so that /usr/local/include comes after them.
• Adjust patch-cmake so that it works correctly with the setup in ports: instead of setting the <LIB>_LAVC variables which assume there is a separate libavcodec installed, make the code fall back to the <LIB>_FF_LAVC ones, which are set when there's a libavcodec inside an ffmpeg tree (our case with ffmpeg0). This also takes care of setting FFMPEG_INCLUDE_DIR with /usr/local/include/ffmpeg0 and passing it in the right place in the command-line.

Tested with both CMake 3.3.1 and 3.4.0.