build: allow custom lldb headers path with npm install

[mmarchini]

It's possible to set a custom lldb headers path if configuring the build manually. This commit also allows users to do it when using npm install. This is useful when using lldb built from source, for example.

[codecov-io]

Codecov Report

Merging #315 into master will decrease coverage by 0.81%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #315      +/-   ##
=========================================
- Coverage   79.21%   78.4%   -0.82%     
=========================================
  Files          33      33              
  Lines        4225    4246      +21     
=========================================
- Hits         3347    3329      -18     
- Misses        878     917      +39

Impacted Files	Coverage Δ
src/llv8.cc	70.35% <0%> (-3.25%)	:arrow_down:
src/llv8.h	80.95% <0%> (-2.39%)	:arrow_down:
src/llv8-inl.h	91.87% <0%> (-1.77%)	:arrow_down:
src/llv8-constants.h	98.48% <0%> (-1.52%)	:arrow_down:
src/llv8-constants.cc	82.1% <0%> (-1.39%)	:arrow_down:
src/llnode.cc	74.4% <0%> (+0.2%)	:arrow_up:

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[mmarchini]

Landed in 9a58d8cf3fa1