ci: make sure we build Git parallel

ci: make sure we build Git parallel

Commit 2c8921db2b (travis-ci: build with the right compiler,
2019-01-17) started to use MAKEFLAGS to specify which compiler to use
to build Git.  A bit later, and in a different topic branch commit
eaa62291ff (ci: inherit --jobs via MAKEFLAGS in run-build-and-tests,
2019-01-27) started to use MAKEFLAGS as well.  Unfortunately, there is
a semantic conflict between these two commits: both of them set
MAKEFLAGS, and since the line adding CC from 2c8921db2b comes later in
'ci/lib.sh', it overwrites the number of parallel jobs added in
eaa62291ff.

Consequently, since both commits have been merged all our CI jobs have
been building Git, building its documentation, and applying semantic
patches sequentially, making all build jobs a bit slower.  Running
the test suite is unaffected, because the number of test jobs comes
from GIT_PROVE_OPTS.

Append to MAKEFLAGS when setting the compiler to use, to ensure that
the number of parallel jobs to use is preserved.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>