build: drop check for ANSI compiler

build: drop check for ANSI compiler

Using automake.git (will become 1.12 someday), I got this error:

configure.ac:90: error: automatic de-ANSI-fication support has been removed
/usr/local/share/aclocal-1.11a/protos.m4:13: AM_C_PROTOTYPES is expanded from...
configure.ac:90: the top level
autom4te: /usr/bin/m4 failed with exit status: 1

In short, pre-C89 compilers are no longer a viable portability
target.  Besides, our code base already requires C99, so worrying
about pre-C89 seems pointless.

* configure.ac (AM_C_PROTOTYPES): Drop, since newer automake no
longer provides it.