update build.sh and CONTRIBUTING.md

zcutil/build.sh

@@ -5,6 +5,12 @@
 
 set -eu -o pipefail
 
+# Check if cmake, a new dependency for randomx support, is installed on system and exits if it is not
+if ! [ -x "$(command -v cmake)" ]; then
+  echo 'Error: cmake is not installed. Install cmake and try again.' >&2
+  exit 1
+fi
+
 function cmd_pref() {
     if type -p "$2" > /dev/null; then
         eval "$1=$2"