travis/osx - parellelize the heavy compile jobs

- We can probably get away with this since OS X
   does not have an OOM killer
 - Will watch to see if it actually helps overall build times...

.travis.yml

@@ -45,11 +45,11 @@ matrix:
       compiler: clang
       # https://docs.travis-ci.com/user/languages/objective-c/#Supported-OS-X-iOS-SDK-versions
       osx_image: xcode7.2 # upgrades clang from 6 -> 7
-      env: JOBS=10 MASON_PUBLISH=true
+      env: JOBS=10 MASON_PUBLISH=true HEAVY_JOBS=3
     - os: osx
       compiler: clang
       osx_image: xcode7.2 # upgrades clang from 6 -> 7
-      env: JOBS=10 COVERAGE=true
+      env: JOBS=10 COVERAGE=true HEAVY_JOBS=3
 
 before_install:
  - export PYTHONUSERBASE=${PYTHONUSERBASE}

Makefile

@@ -7,6 +7,10 @@ ifeq ($(JOBS),)
 	JOBS:=1
 endif
 
+ifeq ($(HEAVY_JOBS),)
+	HEAVY_JOBS:=1
+endif
+
 all: mapnik
 
 install:
@@ -38,7 +42,7 @@ python:
 
 src/json/libmapnik-json.a:
 	# we first build memory intensive files with -j1
-	$(PYTHON) scons/scons.py -j1 \
+	$(PYTHON) scons/scons.py -j$(HEAVY_JOBS) \
 		--config=cache --implicit-cache --max-drift=1 \
 		src/renderer_common/render_group_symbolizer.os \
 		src/renderer_common/render_markers_symbolizer.os \