In bug 922912, we folded back gkmedias.dll info xul.dll, so in practice, there
is no default configuration left that exercises GKMEDIAS_SHARED_LIBRARY. And
sure enough, it's been broken for months in many different ways.
The gkmedias intermediate library is however kept for webrtc signaling tests.
Even though we compile C code as C99, we used to need
-Wdeclaration-after-statement because MSVC didn't allow declarations after
statements.
However, Visual Studio 2013 added support, so we can now merrily mix
declarations and statements everywhere. Hooray.
The patch removes 455 occurrences of FAIL_ON_WARNINGS from moz.build files, and
adds 78 instances of ALLOW_COMPILER_WARNINGS. About half of those 78 are in
code we control and which should be removable with a little effort.
