Tighten up plInBuildTree logic. Test that the current directory is
exactly the same as the top-level build directory or a subdirectory of
that directory. (The old logic did not distinguish say build_dir from
build_dir_install since the first letters of "build_dir_install"
coincided with "build_dir".) The subdirectory logic implies searching
for the path delimiter, so make sure that is appropriately designated
for the different Unix and Windows path delimiters. Finally, make the
whole path name comparisons independent of case on Windows platforms
(as opposed to the old code which only make the drive letter case
Lightly tested on Linux and MinGW/Wine. More comprehensive
testing is requested.