cmake: default the Windows runtime prefix to UCRT64

CMake's x64 build definitions already select UCRT64, but the runtime
fallback still assumes `/mingw64`. Match the existing defaults when
`system_prefix()` cannot infer the prefix from the executable location.
Normal discovery and explicit architecture definitions remain unchanged.

Assisted-by: GPT-6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

contrib/buildsystems/CMakeLists.txt

@@ -258,7 +258,7 @@ add_compile_definitions(PAGER_ENV="LESS=FRX LV=-c"
 			BINDIR="bin")
 
 if(WIN32)
-	set(FALLBACK_RUNTIME_PREFIX /mingw64)
+	set(FALLBACK_RUNTIME_PREFIX /ucrt64)
 	# Move system config into top-level /etc/
 	add_compile_definitions(FALLBACK_RUNTIME_PREFIX="${FALLBACK_RUNTIME_PREFIX}"
 		ETC_GITATTRIBUTES="../etc/gitattributes"