There is no need to build as root, building as a regular user should be preferred.
In the source tree root folder, run:
source build/envsetup.sh lunch replicant_crespo-userdebug
Then, start the build:
parallel_tasks=$(echo "$(grep 'processor' /proc/cpuinfo | wc -l ) + 1" | bc) make -j$parallel_tasks bacon
$parallel_tasks
holds an optimized number of parallel tasks for your hardware. You may want to reduce this number if e.g. the computer runs out of RAM during the build.
out/target/product/crespo/
for the targetout/host/linux-x86/bin/
for the host