boost dependency: grep-able

[ax3l]

@psychocoderHPC just realized the following:

it would be quite neat if we could replace the hard-coded boost min-version in FindmallocMC.cmake by the following logic inside FindmallocMC.cmake:

• find mallocMC
• parse min version (to CMake var mallocMC_BOOST_MIN_VERSION)
• find_package(Boost mallocMC_BOOST_MIN_VERSION REQUIRED)

This PR is to remind that we need, e.g., close to version.h or CMakeList.txt a define that can be queried.

[ax3l]

maybe this can also be solved with cmake config files (?)
@slizzered @erikzenker

[ax3l]

can be moved to 2.3+

[psychocoderHPC]

the boost depedency is removed with #169 so I will close this issue

[ax3l]

nice, for the record, the right way to do this these days would have been with the find_dependency(...) macro:
https://github.com/openPMD/openPMD-api/blob/0.11.1-alpha/openPMDConfig.cmake.in#L26
https://cmake.org/cmake/help/v3.17/module/CMakeFindDependencyMacro.html