add support to build/use libpbf2geojson on OpenBSD (fixes #280)

[landryb]

with the makefile bit, i get this:

[15:39] daytona:/tmp/Vector-Tiles-Reader-QGIS-Plugin/ $
    gmake openbsd
c++ -fPIC -std=c++11 -I/usr/local/include -s -Os -Ofast -O3 --shared -o ./ext-libs/pbf2geojson/pbf2geojson_openbsd_x86_64.so ./ext-libs/pbf2geojson/pbf2geojson.cpp

and i can use the native lib:

2020-03-11T15:17:19     WARNING    Vector Tiles Reader 3.2.2 (Python 3.7.6)
2020-03-11T15:18:05     WARNING    Loading native lib...
2020-03-11T15:18:05     WARNING    Native decoding supported!!! (OpenBSD, 64bit)

@lbartoletti maybe you want the same for FreeBSD ?

[codecov]

Codecov Report

Merging #281 into dev-qgis3 will decrease coverage by 0.07%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##           dev-qgis3     #281      +/-   ##
=============================================
- Coverage      61.22%   61.15%   -0.08%     
=============================================
  Files             17       17              
  Lines           3384     3388       +4     
=============================================
  Hits            2072     2072              
- Misses          1312     1316       +4

Impacted Files	Coverage Δ
plugin/util/mp_helper.py	67.88% <0%> (-2.59%)	⬇️

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[lbartoletti]

Thanks @landryb !
I added support for FreeBSD.

cc @rhurlin for information.