This is done because the one in nixpkgs has several problems and we
don't want to conflict with them fixing those problems:
- not building shared libs
- not building debuginfo (more critical for us due to
#549)
- not setting the correct cmake build type
- not setting the correct cxxflags to build the coroutine library
- not building with clang (required for the coroutine library since gcc
is known to miscompile coroutines *in kj*).
CC: #551
Fixes: #550
Change-Id: Ia5b78dc1809963fdd1a8203b127a216cb575d751
