[monero-project/monero] One build directory per branch/arch. (#4286)

cslashm commented on this pull request.

> all: release-all

— mkdir -p build/debug
— cd build/debug && cmake -D CMAKE_BUILD_TYPE=Debug ../..
+ mkdir -p build/$(subbuilddir)/debug
+ cd build/$(subbuilddir)/debug && cmake -D CMAKE_BUILD_TYPE=Debug ../../../..

ooops, never thought about / in a branch name :D

Ok I will add some quote here and there and test with OMG branch name.(I just mod the Makefile for my own purpose and thought it could be useful for other with too slow machine :p)