Shorten build time.

On x86_64 linux build:

LIBTOOL make -j2  60,82s user 13,98s system 112% cpu 1:06,59 total
        make -j2  56,83s user 12,72s system 110% cpu 1:03,20 total
DOLT    make -j2  44,32s user 11,02s system 108% cpu 51,215 total
        make -j2  42,15s user 11,04s system 106% cpu 50,155 total
Signed-off-by: Rafaël Carré <funman@videolan.org>