Rename library to IrrlichtMt

for clarity and to avoid conflicts
closes #16

scripts/ci-build-mingw.sh

@@ -1,10 +1,10 @@
 #!/bin/bash -e
 
-variant=win32
-[[ "$1" == "--64" ]] && variant=win64
-
 [[ -z "$CC" || -z "$CXX" ]] && exit 255
 
+variant=win32
+[[ "$CXX" == "x86_64-"* ]] && variant=win64
+
 libjpeg_version=2.0.6
 libpng_version=1.6.37
 zlib_version=1.2.11