Remove and/or shorten many comments in configure.ac.

Remove checks for many common Unix programs. Our build process currently
assumes they are there and makes no provisions for any other world-views.
(We can add some of these checks back at some later time if it should prove
useful, but right now, we do not need to check to see whether "rm" exists.)

Remove checks for many common standard C headers and functions. We assume
ISO/ANSI C++, and we always use the <cfoo> versions of ANSI C's <foo.h>
headers, so these checks will not help anything.

Edit configure's warning messages for clarity and content.

Change checks for "optional" programs to default to using "true" instead of
"false", so that a failure to find, e.g., etags, will be less likely to result
in make failing.

No longer shall we check for --enable-purify or --with-purify options.
No longer shall we propagate these to the Makefiles.

configure regenerated using autoconf-2.57.

Please feel free to send me any questions or comments you have. :-)

llvm-svn: 10814