Silence GCC discovery and bail when none available

On some systems, `which` will print to stderr when an argument can't be
found. This will now be suppressed. Also removed `basename` to more
gracefully handle the situation where `g++` is not on the path.

Makefile

@@ -1,6 +1,9 @@
 # When unset, discover g++. Prioritise the latest version on the path.
 ifeq (, $(and $(strip $(CXX)), $(filter-out default undefined, $(origin CXX))))
-  override CXX	:= $(shell basename `which g++-12 g++-11 g++-10 g++-9 g++-8 g++ | head -n 1`)
+  override CXX	:= $(shell which g++-12 g++-11 g++-10 g++-9 g++-8 g++ 2>/dev/null | head -n 1)
+  ifeq (, $(strip $(CXX)))
+    $(error Could not locate the g++ compiler. Please manually specify its path using the CXX variable)
+  endif
 endif
 
 export CXX