Add optional support for using the system libicu

[Omair Majid]

Hi,

webrev:
http://cr.openjdk.java.net/~omajid/webrevs/system-icu/00/

This implements support for linking against the system-installed ICU
instead of building and linking against the bundled copy of ICU in
OpenJDK8. Now someone building OpenJDK can delete the .../font/layout
directory and use configure to build completely against the system
headers and libraries.

I had to move SunLayoutEngine.cpp so the #include would be "layout/..",
which is where the system-installed ICU headers are. It now also matches
the class name (which is sun.font.SunLayoutEngine, not
sun.font.layout.SunLayoutEngine).

I tried to keep all new ICU-related changes next to the existing
freetype-using-logic since they are both text related.

FYI: I am quite sure the build breaks if built using the old build
system; I will fix that next.

Any comments or suggestions?

Thanks,
Omair

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