CMake: Misleading error message in C only projects trying to find Boost

I have multiple CMake library projects here, one of them using Boost threads. One library project ("A") exports its own config together with a `find_dependency(Boost COMPONENTS thread)` call.

Another library project ("B") uses `find_package(A)` and this way the find_dependency is executed which calls `find_package(Boost COMPONENTS thread)` internally. That is then used by a third library ("C"), so you a have a chain of dependencies here.

Problem: library C currently has its LANGUAGES set to `C` in the `project()` call. Linking libB in libC is possible, but since the switch from CMake FindBoost module to cmake package config coming with Boost I get a build error, which I could track down to Boost.

Minimal example requires a short *CMakeLists.txt* only:

```cmake
cmake_minimum_required(VERSION 3.5)
project(bt
VERSION 0.1
LANGUAGES C
)

find_package(Boost REQUIRED CONFIG
COMPONENTS
thread
)
```

The error message is like this:

```
CMake Error at /usr/lib/x86_64-linux-gnu/cmake/BoostDetectToolset-1.74.0.cmake:5 (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 )
CMake Error at /usr/lib/x86_64-linux-gnu/cmake/BoostDetectToolset-1.74.0.cmake:5 (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_atomic-1.74.0/boost_atomic-config.cmake:27 (include)
/usr/lib/x86_64-linux-gnu/cmake/boost_thread-1.74.0/boost_thread-config.cmake:100 (find_package)
/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 )
Performing Test CMAKE_HAVE_LIBC_PTHREAD
Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
Found Threads: TRUE
Configuring incomplete, errors occurred!
See also "/home/adahl/build/tmp/bt/CMakeFiles/CMakeOutput.log".
```

*Note:* this still happens with most recent Boost, it was just easier to reproduce on my host machine like this.

*Note:* this only happens if COMPONENTS are involved.

Analysis: the problem is in *tools/boost_install/BoostDetectToolset.cmake* right in the first line:

```cmake
string(REGEX MATCHALL "[0-9]+" _BOOST_COMPILER_VERSION ${CMAKE_CXX_COMPILER_VERSION})
```

In my case `${CMAKE_CXX_COMPILER_VERSION}` is empty here (C only project), leading to that error message. I would propose to check for that variable before using it and bailout with a meaningful error message if it is found to be empty.