Add support for ICU 4.2

Supporting ICU 4.2 seems useful because it ships with CentOS 6.

Versions before ICU 4.6 don't support pkg-config, so document an
installation method without using pkg-config.

In ICU 4.2, ucol_getKeywordsForLocale() sometimes returns values that
will not be accepted by uloc_toLanguageTag().  Skip loading keyword
variants in that version.

Reported-by: Victor Wagner <vitus@wagner.pp.ru>