The build failures are not because the used compiler is too old but it's missing features: https://sourceforge.net/p/mingw-w64/bugs/959/
https://github.com/geany/geany/pull/3568 changes the used cross compiler toolchain to its "posix" variant which has all necessary C++ features and so the build will succeed.
I added there some more information about the used compiler to the build log.