[CMake] Find_Boost sets Boost_INCLUDE_DIR before making sure the right version is found

[Phil Osteen]

I'm not sure if this is a pattern of some sort or if it is unique to
Find_Boost, but using EXACT with find_package(Boost), as described here
<https://cmake.org/cmake/help/v3.0/module/FindBoost.html>, will fail to
find the correct boost version depending on the location of other boost
versions.

In FindBoost.cmake
<https://github.com/Kitware/CMake/blob/5fdd7d21f432e7a43a0cfa1a26da2d92217c7a60/Modules/FindBoost.cmake#L998>,
a list of potential boost search directories and path suffices are used to
look for boost/config.hpp.

Once _any_ directory that contains boost/config.hpp is found,
Boost_INCLUDE_DIR is set. After which,
${Boost_INCLUDE_DIR}/boost/version.hpp is used to extract the version. If
this version does not match the one passed to find_package, cmake declares
that it cannot find the specified version.

Since find_path <https://cmake.org/cmake/help/v3.0/command/find_path.html>
has a defined search process, if any other directory that happens to be
searched first contains boost/config.hpp, the process will fail, even if
the correct directory would have be subsequently found by find_path.

One approach to resolve this would be to check the version.hpp value after
a find_path call to make sure the correct version was found, and if not, to
re-run find_path with, for example NO_CMAKE_PATH, then if that doesn't
work, with NO_CMAKE_PATH and NO_CMAKE_ENVIRONMENT_PATH, and so on.

I wanted to run this by everybody before writing the patch, perhaps a
cleaner approach could be devised. Cheers

Phil
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