catch missing g++ early

[springmeyer]

appears gcc may be installed but not g++ on ubuntu trusty by default

**** building bcp ****

/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/tools/gcc.jam:148: in gcc.init from module gcc
error: toolset gcc initialization:
error: no command provided, default command 'g++' not found
error: initialized from ../../project-config.jam:12
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/build/toolset.jam:41: in toolset.using from module toolset
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/build/project.jam:1007: in using from module project-rules
../../project-config.jam:12: in modules.load from module project-config
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/build-system.jam:249: in load-config from module build-system
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/build-system.jam:412: in load-configuration-files from module build-system
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/build-system.jam:524: in load from module build-system
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/kernel/modules.jam:289: in import from module modules
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/tools/build/v2/kernel/bootstrap.jam:139: in boost-build from module
/home/ubuntu/llmr-native/mapnik-packaging/osx/out/packages/boost_1_55_0-gcc-x86_64/boost-build.jam:17: in module scope from module

[zuraizm]

@dane Springmeyer I facing the same error