build: change checkheaders to use regular build rules

Many compilers need special flags to compile *.h files as regular
source code, if they will do so at all.  Rather than hoping all
compilers will have such a flag and adding mappings for it, create
wrapper .c files for test building single headers.

This allows using the regular rule for compiling C files without the
need for special flags, and it also provides proper dependency tracking
for these objects.

Signed-off-by: Mans Rullgard <mans@mansr.com>
