Low: using $(MAKE) variable, boost parallel build time

which can be easily around 50% or more, depending on the HW (here
i7-3520M) and the level of parallelism:

$ ./configure; make clean; time make -j3
[...]
make[1]: warning: jobserver unavailable: using -j1. Add `+' to parent

make rule.

[...]
real 1m7.858s
user 0m58.931s
sys 0m5.991s

$ git am Low-using-MAKE-variable-boost-parallel-build-time.patch
$ ./configure; make clean; time make -j3
[...]
real 0m36.716s
user 1m15.494s
sys 0m6.016s

$ git reset HEAD^; git checkout .
$ ./configure; make clean; time make -j3
[...]
make[1]: warning: jobserver unavailable: using -j1. Add `+' to parent

make rule.

[...]
real 1m7.698s
user 0m58.568s
sys 0m6.183s

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>