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

This fixes building/installing into a relative path.

build-llvm.sh

@@ -22,6 +22,9 @@ if [ -z "$PREFIX" ]; then
     exit 1
 fi
 
+mkdir -p "$PREFIX"
+PREFIX="$(cd "$PREFIX" && pwd)"
+
 : ${CORES:=$(nproc 2>/dev/null)}
 : ${CORES:=$(sysctl -n hw.ncpu 2>/dev/null)}
 : ${CORES:=4}