If we use clang to build bpftrace instead of GCC, it will complain about
trying to use the `-std-c++14` flag to comile headers.c. If we force
cmake to skip this flag on `.c` files, clang is not able to link
libresources.a correctly.

This patch changes the resources workflow to generate a .cpp file
instead, and to have a `src/headers.h` with all headers we currently
use. With this approach we get a safer linkage because the same
definition for the header strings is being used in clang_parser.cpp and
in headers.cpp.

Fixes: https://github.com/iovisor/bpftrace/issues/350