vagrant: install gcc-7 on openindiana

[ThomasWaldmann]

otherwise it does not find the C compiler.

also: add a note that we want to have python-37 package (in a comment,
because it is already preinstalled in our base VM image).

[codecov-commenter]

Codecov Report

Merging #5363 into 1.1-maint will decrease coverage by 0.70%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           1.1-maint    #5363      +/-   ##
=============================================
- Coverage      80.21%   79.51%   -0.71%     
=============================================
  Files             27       27              
  Lines          10396    10396              
  Branches        1829     1829              
=============================================
- Hits            8339     8266      -73     
- Misses          1546     1613      +67     
- Partials         511      517       +6     

Impacted Files	Coverage Δ
src/borg/xattr.py	59.61% <0.00%> (-22.60%)	⬇️
src/borg/platform/__init__.py	73.68% <0.00%> (-15.79%)	⬇️
src/borg/platform/base.py	79.43% <0.00%> (-3.74%)	⬇️
src/borg/archive.py	82.23% <0.00%> (-0.90%)	⬇️
src/borg/helpers.py	86.99% <0.00%> (-0.36%)	⬇️
src/borg/repository.py	85.23% <0.00%> (-0.20%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 40b3bca...69d89d7. Read the comment docs.