Avoid rebuilding GUI on every make
configure: Don't check for java when we disable the gui
Install precompiled GUI when java compiler is missing

Also add --disable-precompiled-gui option to disable installing the binary
configure: Add m4 java checks
Merge branch 'ui-build-refinement' into NetworkManager

Conflicts:
	configure.ac
abc-iw: use static functions
abc-nm: Make retrieve_variant assert on the expected type
abc-nm: Move function
abc-nm: Merge functions, *_connect now only does what it says
abc-nm: save active connection
abc-nm: Initialize device path and connection obj on *_init

This was previously done by connect, but removed ca. 2 commits ago
abc-nm: fix bogus code, rename vars for clarity
abc: add iface_is_connected_cb and implement it in abc-nm/abc-iw
abc: query if interface is enabled before enabling
abc-iw: initialize is_enabled with 0, -1 is never used
abc-wifi: Check if connected before connecting
abc-nm: Wait for success on connect call (up to 25s)

Such a long operation probably shouldn't be blocking