Support boost 1.60

[springmeyer]

There are at least two problems preventing upgrading to boost 1.60:

• The boost regex configure check for ICU is broken - this works around 73a2247
• Mapnik also does not work against boost regex at 1.60: Support boost 1.60 mapnik#3333

The two are not necessarily related for sure.

[artemp]

@springmeyer - I'm curious about ^ . I have been using boost 1.60 for a while.

[artemp]

./bootstrap.sh --with-python=python3 --with-libraries=program_options,regex,filesystem,thread,python,timer,date_time,iostreams,test --prefix=/usr/local --with-icu=/usr/local --with-toolset=clang

this is how I build boost with regex+ICU