windows: effectively revert r32449

The _strdup usage in opal/util/basename looks like it was a product of
Windows compatibility (see r11336), which we don't care about any
more.  Further, opal/win32/win_compat.h, which we sitll maintain for
cygwin compatibility, #define's strdup to _strdup (which is what
Microsoft wants you to use).  

So this old _strdup in opal/util/basename.c (and its corresponding
check in configure.ac) should just be removed.