--alternative-gcc-output-dir configure option has been specified,
HAIKU_ADD_ALTERNATIVE_GCC_LIBS is set by default. One has to explicitely
unset it in UserBuildConfig to avoid building the alternative gcc libs.
* Adjusted documentation accordingly.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@29607 a95241bf-73f2-0310-859d-f6bbb57e9c96