mapnik: fix for Boost 1.72

[fxcoudert]

Upstream fix from mapnik/mapnik#4098

[fxcoudert]

Weird build failure on High Sierra:

Exiting... the following required dependencies were not found:
   - boost regex (more info see: https://github.com/mapnik/mapnik/wiki/Mapnik-Installation & http://www.boost.org)�[0m
�[91m
See '/private/tmp/mapnik-20200124-67315-473tjn/mapnik-v3.0.22/config.log' for details on possible problems.�[0m
�[94m
Also, these OPTIONAL dependencies were not found:
   - boost program_options (more info see: https://github.com/mapnik/mapnik/wiki/Mapnik-Installation & http://www.boost.org)
   - boost_regex_icu (libboost_regex built with optional ICU unicode support is needed for unicode regex support in mapnik.)
   - proj (Proj.4 C Projections library | configure with PROJ_LIBS & PROJ_INCLUDES | more info: http://trac.osgeo.org/proj/)
   - gdal (GDAL C++ library | configured using gdal-config program | try setting GDAL_CONFIG SCons option | more info: https://github.com/mapnik/mapnik/wiki/GDAL)�[0m

[stale]

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