unhide the scons command run by make
more consistent memset/memcpy usage
array allocation test
add std::string and std::valarray to array allocation benchmark
fix rendering of multiple styles with OGR plugin - refs #2048 and mapbox/tilemill#2202
fix #2141
check filesystem before trying to open plugin + only report unique directories searched - closes #2131

Conflicts:
	src/datasource_cache.cpp
only return true if plugins are actually newly registered
iwyu
Merge branch 'master' of github.com:mapnik/mapnik into expr-v2