build: Allow providing extra libs for Boost Chrono

Pass BOOST_CHRONO_EXTRALIBS to give extra dependency
libraries for Boost Chrono which are not auto-detected.
build: Pass BOOST_CPPFLAGS to sleep implementation test

Makes sure that `-pthread` and other flags required for boost
compilation are passed.
gitian: Reduce build time for boost windows dependency

Build only the mt-s (multithreaded, static runtime link)
variant of the library.

This cuts the build time in half.

Also prevent unpacked boost from ending up in output directory,
checking and hashing all the source files as output files this
takes a lot of time and is pointless.
gitian: Build boost dependency for linux

Instead of using the boost provided by Ubuntu 12.04, build our own
dependency like we do for Windows.

This allows using a much newer version (1.55 versus 1.46) as well as
building with `-fPIC` so that `-pie` can be used in the x86-64 build.
gitian: Add openssl to linux deps

Build OpenSSL instead of using distribution-provided
library.