Fixes Makefile portability

Compiler is referenced by `$(CC)` from recipes. RE2C no longer
produces `*.y` files because they confused Automake - `*.yy` is used
instead. To support general transformation from `*.yre` through `*.yy`
to `*.{c,h}` implicit rules have been introduced. However, this
creates a problem with Lemon because it relies on current working
directory quite heavily, so custom distclean hook has been introduced
for cleaning up.

Another significant change is that resulting `*.c` and `*.h` files are
not part of the distribution. The reasoning is that what be safely
generated, should not be distributed, which is of course subjective
approach.

The source code tree passes `make distcheck` but only after previous
invocation `make` compilate target (i.e., the default one).

This commit fixes megous/libxr#9.