Use C instead of C++ for fuzzer targets
- configure.ac: Remove AC_PROG_CXX
- fuzz/Makefile.am: Amend *_SOURCES for .c fuzzer targets, Remove non-portable EXTRA_DIST wildcards, Add dist-hook to include fuzzer target files, Add oss-fuzz make target.
- fuzz/fuzzer.h: New file
- fuzz/*cc: Rename to C, include fuzzer.h
- fuzz/main.c: Include fuzzer.h
- fuzz/run-clang.sh: Build and run fuzzer targets made with clang/libFuzzer