Remove AM_C_PROTOTYPES from configure.in, which was dropped in automake
1.12 as pre-ANSI C compilers are no longer worth caring about.
Add fails_with clang. AGG tries to return a const reference as a
non-const reference, which is allowed by gcc and llvm but not clang 3.1.
Signed-off-by: Adam Vandenberg <flangy@gmail.com>