scons: Control caching via new SCONS_CACHE_DIR environment variable.

This serves several purposes:
- disable caching in situations were is it useless or undesired
- share caches among all trees
- simplify purging the cache (when it's a single location)
- move the cache out of the tree, since that slows downs IDEs considerably

To retain previous behavior just define do

  export SCONS_CACHE_DIR=$PWD/build/cache

before invoking scons.

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