don't pollute include dir with installed submodules

[Vedingrot]

When install mapnik with bundled submodules it creates mapbox and
protozero directories which could conflict with previously installed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.76%. Comparing base (09067f5) to head (a8d6532).
Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4452   +/-   ##
=======================================
  Coverage   73.75%   73.76%           
=======================================
  Files         525      525           
  Lines       33532    33532           
  Branches     4139     4140    +1     
=======================================
+ Hits        24733    24736    +3     
+ Misses       8797     8794    -3     
  Partials        2        2           

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[hummeltech]

I see what you mean, but if one already has those dependencies installed, wouldn't they just build Mapnik with the corresponding CMake options?

mapnik/CMakeLists.txt

Lines 38 to 41 in a8d6532

	mapnik_option(USE_EXTERNAL_MAPBOX_GEOMETRY "Use a external mapnik/geometry.hpp. If off, use the submodule" OFF)
	mapnik_option(USE_EXTERNAL_MAPBOX_POLYLABEL "Use a external mapnik/polylabel. If off, use the submodule" OFF)
	mapnik_option(USE_EXTERNAL_MAPBOX_PROTOZERO "Use a external mapnik/protozero. If off, use the submodule" OFF)
	mapnik_option(USE_EXTERNAL_MAPBOX_VARIANT "Use a external mapnik/variant. If off, use the submodule" OFF)

[Vedingrot]

For example, include/mapnik may contains unsupported version of
dependencies that requires by other programs. Using submodules solves
this problem but they should be installed in a directory that "belongs"
only for mapnik because they will be used only by it.

[mathisloge]

I don't think that this is really needed, since you should install mapnik in a extra dir, if you have conflicting versions of those, since you should never link those bits together.

I do have concernce regarding the integration, since the headers now need to be included as <mapnik/protozero/...> etc.

[Vedingrot]

There is compiler flag -I"${includedir}/mapnik that has been added
in https://github.com/mapnik/mapnik/pull/4441/files so there's no
need to change headers.