Rename configure.in as suggested by autoconf.

The configure.in name is deprecated.  Autoconf suggests naming its
input file configure.ac, so let's stick with the standard.
Add some missing checks spotted by autoscan.

Also explicitly specify the AC_CONFIG_AUX_DIR.
Switch to autoreconf invocation.

Replace autoconf with autoreconf in Travis script and README, as
recommended for the autotools suite.  This will also regenerate
config.h.in using autoheader, so remove it from the repository.
Let autoconf handle the compiler C99 mode.

Use the autoconf-provided @CC@ substitution as well as @CFLAGS@ and
@LDFLAGS@ in Makefile.in.
Remove explicit -O3 flag.

This should be set using configure, so remove the explicit default
value.
Port the tests/ directory to use autoconf.

Use autoconf's @CC@ and @CFLAGS@ configuration in Makefile.in.
Ignore file config.h.in generated by autoheader.
Move AC_CONFIG_HEADERS toward the end.

Output of config.h should be listed along with the other output files.
Merge pull request #22 from acolomb/cleanup-autoconf

Clean up autoconf build system