AC_CONFIG_AUX_DIR and Windows

[Eric Blake]

The automake testsuite recently introduced a portability problem when it used 
AC_CONFIG_AUX_DIR([aux]) in a couple of tests.  Unfortunately, Windows reserves 
aux as a special filename, so you cannot use a subdirectory named aux for this 
purpose; these tests needlessly fail under cygwin and other Windows ports.

The name of the AC_CONFIG_AUX_DIR macro is somewhat unfortunate for 
portability, because a naive user is likely to choose "aux" as was done in the 
automake testsuite.  You should consider either renaming the macro, or updating 
the macro to error out (or at least warn) if user tries to name their auxiliary 
directory "aux" on the grounds that their project will not port to cygwin, 
mingw32, or djgpp without changes.  Also, the "File System Conventions" section 
of the manual should be updated to list all known filenames that will not port 
to Windows.  This includes, but is not limited to, aux, com1, com2, lpt1, nul 
(the equivalent of Unix's /dev/null, but not tied to an absolute path), these 
special filenames with a suffix (such as aux.sh), and so forth.