Tidy up autotooling.

Makefile.am: Remove AUTOMAKE_OPTIONS as these options are set in configure.ac.
Makefile.am: Remove -std=c99 and -std=gnu99; request a C99 compiler instead.
Makefile.am: Remove repeated _LDADD and _CPPFLAGS declarations.
configure.ac: Request C99 compiler.
configure.ac: Remove tests for dinosaur compilers (their results weren't used
  anyway).
configure.ac: Turn on silent rules, if available.

Regenerate autocruft.

Makefile.in: Regenerate.
aclocal.m4: Regenerate.
autom4te.cache/requests: Regenerate.
configure: Regenerate.