core/erlc.mk: Support non-US-ASCII characters in paths

[dumbbell]

Without this patch, the added testcase would fail with the following error:

gmake[3]: *** No rule to make target '(...)/erlang.mk/test/h��test_core_makedep_non_usascii_paths/deps/test_core_makedep_non_usascii_paths_dep/include/hello.hrl', needed by 'src/hello.erl'.  Stop.

In this case, the path contains UTF-8-encoded é characters and they appear to be converted to ISO-8859-1 in the makedep.erl script.

[dumbbell]

My initial patch was way too complicated. After re-reading the Using Unicode in Erlang documentation, I understood that the conversion was done by the file:write_file() function. Converting the string just before passing it to the file module is enough to fix the problem.

[essen]

Nice!

[essen]

Merged, thanks! Push in a few.