[build] Fix $(RUNTIME) initialization

Commit c7447ed is broken.

`$(RUNTIME)` needs to contain a managed runtime that can be used to
execute unit tests and other build utilities:

	RUNTIME := $(shell if [ -f `which mono64` ] ; then echo mono64 ; else echo mono; fi)

Unfortunately on some machines this doesn't evaluate as intended:
if `mono64` isn't in `$PATH` then it is equivalent to:

	if [ -f ] ; then
		echo mono64
	else
		echo mono
	fi

`if [ -f ]` is always true (!), so `mono64` is used even when it
doesn't exist (the whole point for the check in the first place!).

Quote the value of `which mono64` so that even if **which**(1)
returns the empty string it still behaves properly:

	RUNTIME := $(shell if [ -f "`which mono64`" ] ; then echo mono64 ; else echo mono; fi)

This quoting ensures that `-f` *always* has a value *other than `]`*,
which allows `$(RUNTIME)` to have the correct value.

Author: jonpryor

Makefile

@@ -9,7 +9,7 @@ ifeq ($(OS),Linux)
 NATIVE_EXT = .so
 endif
 
-RUNTIME       := $(shell if [ -f `which mono64` ] ; then echo mono64 ; else echo mono; fi) --debug=casts
+RUNTIME       := $(shell if [ -f "`which mono64`" ] ; then echo mono64 ; else echo mono; fi) --debug=casts
 
 XA_CONFIGURATION  = XAIntegrationDebug