Resolve $PREFIX to an absolute path at the start of shell scripts

This fixes building/installing into a relative path.

build-mingw-w64.sh

@@ -14,6 +14,10 @@ if [ -z "$PREFIX" ]; then
     echo $0 [--skip-include-triplet-prefix] dest
     exit 1
 fi
+
+mkdir -p "$PREFIX"
+PREFIX="$(cd "$PREFIX" && pwd)"
+
 if [ -z "$HOST" ]; then
     # The newly built toolchain isn't crosscompiled; add it to the path.
     export PATH=$PREFIX/bin:$PATH