configure: force the use of a C99 compiler

This code base uses C99 constructs frequently (e.g., we use "struct
foo bar { .baz = yow }" kinds of initialization all over the place),
so force the compiler to be C99.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #367 from jsquyres/pr/we-is-c99

configure: force the use of a C99 compiler