build: i18n: update with review comments

Updated according to review comments.

Renamed iculslocs.cpp to iculslocs.cc.

Removed a spurious .gitignore line

Rewrote parts of the configure script.

iculslocs.cc passes cpplint except for the following three errors,
so I excluded it. `UErrorCode &status` is an in-out parameter used
for ICU error reporting.
http://userguide.icu-project.org/design#TOC-Error-Handling

tools/icu/iculslocs.cc:107:  Is this a non-const reference? If so, make const or use a pointer.  [runtime/references] [2]
tools/icu/iculslocs.cc:129:  Is this a non-const reference? If so, make const or use a pointer.  [runtime/references] [2]
tools/icu/iculslocs.cc:160:  Is this a non-const reference? If so, make const or use a pointer.  [runtime/references] [2]