improved config/Makefile.linux-mingw* to support any host easily

Hi, some days ago I tried to build GLEW for my Linux MinGW environment.
I think you already know, gcc has many patterns of prefix, for example "i386-mingw32msvc" "i586-pc-mingw32" "i686-w64-mingw32". And I useing "i686-pc-mingw32" then I forced CC and LD when make, but I think it is not smart.
Then, I edited config file to fix it.
If you will to add this to your project, I'll be grad to contribute.