cerbero: Use separate prefixes for MSVC/MinGW in the MSI

Before we would output mingw, msvc, msvc-debug into the same prefixes, and would set the same env var for all:

C:\gstreamer\1.0\x86_64
C:\gstreamer\1.0\x86
GSTREAMER_1_0_ROOT_X86_64
GSTREAMER_1_0_ROOT_X86

Now we will output into:

-v novisualstudio (default)
C:\gstreamer\1.0\mingw_x86_64
C:\gstreamer\1.0\mingw_x86
GSTREAMER_1_0_ROOT_MINGW_X86_64
GSTREAMER_1_0_ROOT_MINGW_X86

-v visualstudio
C:\gstreamer\1.0\msvc_x86_64
C:\gstreamer\1.0\msvc_x86
GSTREAMER_1_0_ROOT_MSVC_X86_64
GSTREAMER_1_0_ROOT_MSVC_X86

-v visualstudio,vscrt=mdd
C:\gstreamer\1.0\msvc-debug_x86_64
C:\gstreamer\1.0\msvc-debug_x86
GSTREAMER_1_0_ROOT_MSVC_DEBUG_X86_64
GSTREAMER_1_0_ROOT_MSVC_DEBUG_X86

Fixes #281 (closed)