build: use ninja for cmake packages by default

build: use ninja for cmake packages by default

Speed goes from:

Executed in     178.08 secs     fish            external
usr time        20.16 mins      509.00 micros   20.16 mins
sys time        2.88 mins       39.00 micros    2.88 mins

To:

Executed in     175.90 secs     fish            external
usr time        20.19 mins      0.00 micros     20.19 mins
sys time        2.85 mins       497.00 micros   2.85 mins

Tested with "time make -j 12" on AMD Ryzen 3600

When building individual packages, the build time difference is often
significantly bigger than that.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Rosen Penev <rosenp@gmail.com>