package/boost: disable boost-locale for static only build with icu

Boost fails to build with the following error:

error: Tried to build the target twice, with property sets having
error: these incompatible properties:
error:
error:     -  <runtime-link>static <warnings>all
error:     -  <runtime-link>shared <warnings>on

when the following conditions are met:

 - BR2_STATIC_LIBS=y
 - BR2_PACKAGE_ICU=y
 - BR2_PACKAGE_BOOST_LOCALE=y
 - Another BR2_PACKAGE_BOOST_xyz option is enabled, which enables a
   feature not provided just by header files, but that requires
   building a library.

In such a situation, Boost absolutely wants to build the libboost
libraries as shared libraries. Not having boost-locale, or not having
icu is sufficient to avoid the issue.

So, as a simple work-around, we prevent from building boost-locale
when icu and static linking are used.

Fixes:

  http://autobuild.buildroot.net/results/c8f7aa85f5791d8ae8cf4b9085788adc5152286f/



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Thomas:
 - only disable boost-locale when icu is enabled
 - improve commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>