When CrossCompiling=YES or Stage1Only=YES, building the haddocks and the
User's Guide should be skipped, because haddock and mkUserGuidePart
depend on the GHC API.
See Note [No stage2 packages when CrossCompiling or Stage1Only] for details.
There are several places in the build system where the variables
HADDOCK_DOCS and BUILD_SPHINX_* are checked. Instead of also checking
for the variables CrossCompiling or Stage1Only in all those places,
make will now exit with a nice error message when the user requests