...as stoc/Library_javavm.mk depends on it since
9b09a217 "Resolves: #i86470# Wrong Java locale
when using 'nl' and 'fr'".  The i18nlangtag lib in turn depends on libs from
external/liblangtag and external/icu, so those needed to be moved to URELIB,
too.

On Windows, the external icu package was already split into icu and icu_ure
(because "libxml2 is in URE and depends on icuuc*.dll on Windows"), so use that
splitting on all platforms.  (However, the corresponding changes that were
necessary in RepositoryExternal.mk suggest that they had been missing for the
split Windows case until now, and things had happened to work by accident?)

On macOS, a library's install name reflects its (URELIB, OOO, ...) layer, and in
external/icu/icu4c-build.patch there is only a single place to set that for all
libs from external/icu.  This patch changes that from OOO to URELIB, but for the
icui18n lib that should stay at OOO.  The hack to make it URELIB nonetheless
works for now.  To clean this up again, either the whole of icu could go into
URE (dropping the icu vs. icu_ure package split completely), or the macOS layers
URELIB and OOO could be combined into one (as the libs end up in the same
directory anyway).

Change-Id: Idc262fa41481d06ba2cae86ad7629cdccb392c07
Reviewed-on: https://gerrit.libreoffice.org/30272Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>