Fix some porting issues to make everything work on Debian 8 "Jessie"
and Ubuntu 14 "Trusty":

- set cmake minimum version to 2.8.12, this is the Ubuntu 14 version

- combine C and C++ flags and add all of them using
  ADD_COMPILE_OPTIONS() without using generator expressions, which is a
  cmake 3.2 or so feature

- add -fpermissive to force some non-const type casts to compile on
  older versions of gcc

- add -std=c++11 for gcc to enable support on older versions of gcc

- check that the compiler supports -fstack-protector-strong before
  adding it, older versions of gcc do not

- fix the debian section of ./installdeps to include libpng-dev instead
  of libpng16-dev and add gettext for msginit etc.

- fix compat checks in src/common/ffmpeg.cpp and src/wx/cmdevents.cpp to
  check for libavcodec >= 56 instead of > 56, the Debian Jessie version
  is exactly version 56 . With the one exception of
  AV_CODEC_FLAG_GLOBAL_HEADER which is defined in later versions.