icinga-installer: statically link MSVC runtime library on CMake 3.15+

CMake 3.15 introduced the `MSVC_RUNTIME_LIBRARY` property as a way to specify
which MSVC runtime library is used and how it is linked. Also with that
release, policy CMP0091 was introduced where the new behavior no longer adds
the corresponding compile flags to the `CMAKE_<LANG>_FLAGS_<CONFIG>` variables.

The new policy was enabled by 7f164bd,
resulting in the MSI no longer working on previous Windows Server versions
(2019 for example) as the CMake configuration replaced those compile flags
(lines 3-9 in the same file) which no longer works when they are no longer part
of the string. This commit fixes this regression by setting the
`MSVC_RUNTIME_LIBRARY` property on the icinga-installer target.

I've also added a comment stating my understanding of why this is necessary.
Unfortunately, I couldn't find an explanation of why replacing the /MD with the
/MT flag was done originally in the project history, so it's a bit of
guesswork.

https://cmake.org/cmake/help/latest/policy/CMP0091.html
https://cmake.org/cmake/help/latest/prop_tgt/MSVC_RUNTIME_LIBRARY.html

Author: julianbrost

icinga-installer/CMakeLists.txt

@@ -19,6 +19,10 @@ set_target_properties(
   FOLDER Bin
   OUTPUT_NAME icinga2-installer
   LINK_FLAGS "/SUBSYSTEM:WINDOWS"
+
+  # Use a statically-linked runtime library as this binary is run during the installation process where the other DLLs
+  # may not have been installed already and the system-provided version may be too old.
+  MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>"
 )
 
 target_link_libraries(icinga-installer shlwapi)