Allow optional libs detection when cross-compiling

[fidomax]

[codecov[bot]]

Codecov Report

Merging #4207 (cf9bde7) into master (c8b8b5c) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4207   +/-   ##
=======================================
  Coverage   71.47%   71.47%           
=======================================
  Files         445      445           
  Lines       23145    23145           
=======================================
  Hits        16542    16542           
  Misses       6603     6603           

Impacted Files	Coverage Δ
include/mapnik/agg/render_polygon_pattern.hpp	100.00% <ø> (ø)
include/mapnik/agg_helpers.hpp	0.00% <ø> (ø)
include/mapnik/agg_pattern_source.hpp	100.00% <ø> (ø)
include/mapnik/agg_rasterizer.hpp	0.00% <ø> (ø)
include/mapnik/agg_render_marker.hpp	88.46% <ø> (ø)
include/mapnik/agg_renderer.hpp	100.00% <ø> (ø)
include/mapnik/attribute.hpp	0.00% <ø> (ø)
include/mapnik/attribute_collector.hpp	0.00% <ø> (ø)
include/mapnik/attribute_descriptor.hpp	91.66% <ø> (ø)
include/mapnik/boolean.hpp	100.00% <ø> (ø)
... and 166 more

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