Modernizing autotools and assuming C89 C library

[Mike Gran]

Hello Guile Devel,

I pushed a git branch: wip-modernize-autotools.

It removes some obsolete tests by presuming we at least have a
C89-compliant C library. It also uses LT_INIT for libtool, and removes
some macros that autoconf has deprecated.

I thought about continuing on by presuming that we have a
C99-compliant C library, but, I figured I'd stop at C89/C90 and touch
base first.

Since most of these header and function tests are for ancient or odd
architectures, I tested it on Cygwin and MinGW. (Note that the main
branch compiles on MinGW but doesn't run.  So I only tested that it
compiles.)

Let me know what you think.

Thanks,
Mike Gran