c++ tests: only source mapnik-settings.env when it exists

run_tests

@@ -3,7 +3,16 @@
 failures=0
 
 source ./localize.sh
-source mapnik-settings.env
+
+# mapnik-settings.env is an optional file to store
+# environment variables that should be used before
+# running tests like PROJ_LIB, GDAL_DATA, and ICU_DATA
+# These do not normally need to be set except when
+# building against binary versions of dependencies like
+# done via bootstrap.sh
+if [[ -f mapnik-settings.env ]]; then
+    source mapnik-settings.env
+fi
 
 ./test/unit/run 
 failures=$((failures+$?))