add build option to reuse a previous precompile statement file

[KristofferC]

Often, regenerating all precompile statements from scratch is not needed and kind of annoying. During print out of the timing stats for precompile generation, e.g.:

Generating precompile statements... 1083 generated in  96.365000 seconds (overhead  80.354744 seconds)

the "overhead" time is the time spent running everything that generates the statements. The rest of the time is actually including them, which is typically significantly shorter.

This PR saves the precompile statements from a build in a persistent file and one can with a setting in Make.user (JULIA_REUSE_PRECOMPILE=1) chose to use that existing file instead of regenerating a new one. If using the old precompile file fails, a new one will be generated. So reusing the previous one is a bit quicker:

Trying to reuse precompile statements from "C:\\Users\\Kristoffer\\julia\\contrib\\precompile_statements.jl"
1083 precompile statements loaded in  16.833387 seconds

[mbauman]

Is there any reason why this couldn't just be the default behavior?

[vchuravy]

Would be great to have! Especially on slow systems or while debugging system image creation.

[KristofferC]

Rebuilt julia far too many times recently. So I rebased this. Saves around a minute per syimage build.

[martinholters]

Coincidentally came across this while the last line in my terminal reads "Generating precompile statements...". Can we get this forward? Would be really nice.

[ViralBShah]

Is this worth updating, or can it be closed?

[IanButterworth]

I think this can be closed now that this process is parallelized.