i#638: set preprocessor location and flag on MSYS2 MinGW same as on Unix

i#638: Preprocessor location and flags on MSYS2 MinGW are same as on Unix-like system. This is just a second step toward building with MinGW.

Issue: https://github.com/DynamoRIO/dynamorio/issues/638 Signed-off-by: ofry tim4job@bmail.ru