[RT #54703] Don't hardcode /cygdrive

'make test' in blead perl is producing the following failure:

cpan/ExtUtils-MakeMaker/t/MM_Cygwin............................#
Failed test ''/c/WINDOWS/system32/cmd.exe' should be executable"'
FAILED at test 12

Since maybe_command() in MM_Cygwin.pm is hardcoded to look for
'/cygdrive', the test fails if the user changes the mount point prefix
from '/cygdrive' to something else using 'mount -c ...'.  (For example,
I use 'mount -c /'.)

The attached patch fixes this finding the mount point prefix and using
it to determine if Unix or Win32 settings should be used.

Signed-off-by: Chris 'BinGOs' Williams <chris@bingosnet.co.uk>