Moved linux dependent components to HAVE_LINUX section

With this, folly builds cleanly on OSX with the following:

autoreconf -f -i;
export DOUBLE_CONVERSION_PATH=$(pwd)/double-conversion;
LDFLAGS=-L$DOUBLE_CONVERSION_PATH CPPFLAGS="-I$DOUBLE_CONVERSION_PATH -DGTEST_USE_OWN_TR1_TUPLE=1" CC=gcc-4.9 CXX=g++-4.9 ./configure;
DYLD_LIBRARY_PATH=$DOUBLE_CONVERSION_PATH make

Time allowing, I can dive into the components one by one to see if it makes sense to make it multi-platform.