Add helpful error message in case CMAKE_CXX_COMPILER_VERSION is not set

[LeSpocky]

CMake bails out with an error message giving no clue what's going on, if variable CMAKE_CXX_COMPILER_VERSION is not set:

CMake Error at /usr/lib/x86_64-linux-gnu/cmake/BoostDetectToolset-1.74.0.cmake:9 (string):
  string sub-command REGEX, mode MATCHALL needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  /usr/lib/x86_64-linux-gnu/cmake/boost_thread-1.74.0/boost_thread-config.cmake:27 (include)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:141 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:258 (boost_find_component)
  CMakeLists.txt:7 (find_package)

-- Boost toolset is unknown (compiler  )

The added message still leads to a CMake error, but gives the user a better clue where to look for in her own project trying to find Boost.

Output afterwards:

CMake Error at /usr/lib/x86_64-linux-gnu/cmake/BoostDetectToolset-1.74.0.cmake:6 (message):
  CMAKE_CXX_COMPILER_VERSION is not set!
Call Stack (most recent call first):
  /usr/lib/x86_64-linux-gnu/cmake/boost_thread-1.74.0/boost_thread-config.cmake:27 (include)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:141 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:258 (boost_find_component)
  CMakeLists.txt:7 (find_package)

CMake Error at /usr/lib/x86_64-linux-gnu/cmake/BoostDetectToolset-1.74.0.cmake:8 (string):
  string sub-command REGEX, mode MATCHALL needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  /usr/lib/x86_64-linux-gnu/cmake/boost_thread-1.74.0/boost_thread-config.cmake:27 (include)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:141 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:258 (boost_find_component)
  CMakeLists.txt:7 (find_package)

-- Boost toolset is unknown (compiler  )

Fixes: #76

[LeSpocky]

Not sure whether message(FATAL_ERROR "blabla") would make more sense here, or a whole different approach to compiler version detection? Let me know.

[LeSpocky]

if(DEFINED … would have been better?