Only allow running ./installdeps if $MSYSTEM is MINGW32 or MINGW64, that
is, the user started a MINGW 32 bit or 64 bit shell and **NOT** an MSYS
shell.
Use the value of $MSYSTEM to determine if 32 bit or 64 bit target deps
should be installed.
Update usage() text to reflect this.
MISC: rename generic_build_instructions() to build_instructions() .