Make build reproducible

[sthibaul]

eade31a ("Honor environment variables passed at ./configure") made
speech_tools record build flags in user_flags.mak. This however make the
build less reproducible by recording flags from the environment. Also,
external applications using it do not actually need to get flags from
that, they can just inherit them from their own environment. Actually,
I'd even argue that this is preferrable over seeing speech_tools
hardcode flags that applications using its build system will then have
to use.

[lenzo-ka]

I'm hoping to get to this shortly... I'd like to test and get a couple other things in first and make sure we don't have conflicts, but this is a good thing to do!