Add --ccflags option to allow custom compiler flags.

[NHDaly]

Allows the user to specify compiler flags to be passed to cc for
building the shared object and the final executable.

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@lucatrv: I'm not sure about the exact name of this flag. Maybe it should just
be cflags? Or maybe extra_cflags or something?

Thanks!

[lucatrv]

I would consider:

"--cc-flags"
arg_type = String
default = nothing
metavar = "<flags>"
help = "pass custom flags to system compiler when building shared library or executable"

Then in static_julia.jl please pass cc_flags to the julia_flags function, so it creates the whole flags list.

[lucatrv]

One reason for --cc-flags is because I am already adding also a --cc flag for custom system compiler, and I think they would list well together.

[codecov-io]

Codecov Report

Merging #41 into master will decrease coverage by 0.13%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   69.93%   69.79%   -0.14%     
==========================================
  Files           5        5              
  Lines         296      298       +2     
==========================================
+ Hits          207      208       +1     
- Misses         89       90       +1

Impacted Files	Coverage Δ
src/system_image.jl	83.87% <ø> (ø)	⬆️
src/static_julia.jl	77.03% <83.33%> (-0.41%)	⬇️

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[NHDaly]

Sounds good to me! I've updated to cc-flags, moved the logic to the julia_flags() function, updated the help string on static_julia(), and added a unit-test specifically covering --cc-flags. :)

PTAL. thanks again!