Add building and testing of Math C99 functions with CMake

[mborland]

Much like vcpkg has in their patch file, the user needs to define BOOST_MATH_BUILD_WITH_LEGACY_FUNCTIONS to have these files generated. This allows the current install behavior to be retained, and also allows a user to install all 6 of the C99/TR1 targets if they so choose. Compiled targets require Boost.Config so if we changed the default behavior to build all of these targets, it would require all the header only users to define a macro to stop such an action instead.

Closes: #1329

[mborland]

Everything looks good using the Boost CMake standard install and subdir tests. Once in I can add a note in the Boost CMake library specific configuration variables section that.

Are you good with this change @jzmaddock? All the new behavior is behind the BOOST_MATH_BUILD_WITH_LEGACY_FUNCTIONS macro so nothing changes for current consumers, or the boost super project. Between 4 and 6 C99/TR1 targets (based on automatic ldbl support detection) are built like they are with B2, and then the user can link against boost_math_c99 and boost_math_tr1 following the same naming conventions as B2.