gitweb: Makefile improvements

Adjust the main Makefile so you can simply run

     make gitweb

which in turn calls gitweb/Makefile.  This means that in order to
generate gitweb, you can simply run 'make' from gitweb subdirectory:

     cd gitweb
     make

Targets gitweb/gitweb.cgi and (dependent on JSMIN being defined)
gitweb/gitweb.min.js in main Makefile are preserved for backward
compatibility.

Signed-off-by: John 'Warthog9' Hawley <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>