* configure (fn_dirname): New.
Use it to ensure the creation of proper symlinks to Makefiles.
(config.mak): Define top_builddir and top_srcdir.
(CPPFLAGS): Be sure to find the headers.
* Makefile, lib/Makefile, tests/Makefile, tests2/Makefile: Adjust
to set VPATH properly.
Fix confusion between top_builddir and top_srcdir.
Use @BINDIR@ in shebang of examples to put the right path on the target
system. That is, use #!/usr/local/bin/tcc if tcc is installed in /usr/local/bin
and #!/usr/bin/tcc if tcc is installed in /usr/bin/tcc.