Stephan Sundermann authored Jun 21, 2015 and Andrés G. Aragoneses committed Jun 21, 2015

The default CallingConvention is WinAPI, which defaults to
the system's convention which might either be StdCall or CDecl.
For C libraries the CallingConvention to use is CDecl
else strange runtime errors might occur because of stack
corruption.

Since Banshee is using C libraries only, specify the CDecl
CallinvConvention everywhere.

On Windows the MDA in Visual Studio would detect a wrong
calling convention (mono has no such detection).

(We take this opportunity to use const names for libraries
in some DllImport calls which point to the same lib.)

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>