Meta: Do not spam "<compiler-name>: command not found"

In bd7d01e9, Meta/serenity.sh started checking cc and cxx if CC or CXX
respectively are not set in the environment. My machine does not have
cxx symlink in PATH, so every invocation of the script resulted in
the "cxx: command not found" message outputted to stderr, and that
became quite annoying.

Meta/find_compiler.sh

@@ -9,7 +9,7 @@ is_supported_compiler() {
     fi
 
     local VERSION=""
-    VERSION="$($COMPILER -dumpversion)" || return 1
+    VERSION="$($COMPILER -dumpversion 2> /dev/null)" || return 1
     local MAJOR_VERSION=""
     MAJOR_VERSION="${VERSION%%.*}"
     if $COMPILER --version 2>&1 | grep "Apple clang" >/dev/null; then