On Cygwin, use a Cygwin-style path for /bin/install's destination
cygwin's /bin/install doesn't set file modes correctly if the
destination path is a C: style path:
$ /bin/install -c -m 644 foo /cygdrive/c/cygwin/home/ian/foo2
$ /bin/install -c -m 644 foo c:/cygwin/home/ian/foo3
$ ls -l foo*
-rw-r--r-- 1 ian None 0 2011-01-06 18:28 foo
-rw-r--r-- 1 ian None 0 2011-01-06 18:29 foo2
-rwxrwxrwx 1 ian None 0 2011-01-06 18:29 foo3
This causes problems for bindisttest/checkBinaries.sh which then
thinks that e.g. the userguide HTML files are binaries.
We therefore use a /cygdrive path if we are on cygwin