build: fix VPATH builds

* 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.

README

@@ -35,6 +35,16 @@ Documentation:
    make test
    make install
 
+Alternatively, VPATH builds are supported: you may use different
+directories to old build objects, kept separate from your source tree:
+
+   mkdir _build
+   cd _build
+   ../configure
+   make
+   make test
+   make install
+
 By default, tcc is installed in /usr/local/bin.
 ./configure --help  shows configuration options.