Windows: work around a boost 1.70 and cmake SNAFU

Boost got upgraded to 1.70.0 in MSYS2. As a result, cmake (v3.13.4 as of this commit) can not configure boost properly, and cmake configuration fails as a result. This is a workaround as per https://gitlab.kitware.com/cmake/cmake/issues/18865

CMakeLists.txt

@@ -878,6 +878,7 @@ set(OLD_LIB_SUFFIXES ${CMAKE_FIND_LIBRARY_SUFFIXES})
 if(STATIC)
   if(MINGW)
     set(CMAKE_FIND_LIBRARY_SUFFIXES .a)
+    set(Boost_NO_BOOST_CMAKE ON)
   endif()
 
   set(Boost_USE_STATIC_LIBS ON)