Allow overriding default llvm and msvcrt versions.

build-mingw-w64.sh already provides --with-default-msvcrt option, but
this makes it more convenient to use build-all.sh with non-default
msvcrt config.

build-mingw-w64.sh

@@ -3,7 +3,7 @@
 set -e
 
 DEFAULT_WIN32_WINNT=0x601
-DEFAULT_MSVCRT=ucrt
+: ${DEFAULT_MSVCRT:=ucrt}
 unset HOST
 
 while [ $# -gt 0 ]; do