Put quotes around CONDOR_PLATFORM #3555

Fixes the following build issue reported by Igor Sfiligoi on
htcondor-devel:

	Hi all.

	I have just checked out the master branch,
	and while runnnig
	[1528] sfiligoi@uaf-3 ~/dev/condor$ ./configure_uw

	I get this:
	...
	-- Looking for include file X11/Xlib.h - found
	-- DEBUG: Threaded functionality only enabled in Linux, Windows, and Mac
	-- DEBUG: GLIBC_VERSION=25
	-- DEBUG: **TO UW: IF YOU WANT CUSTOM SETTINGS ADD THEM HERE**
	-- ********* Building with UW externals *********
	-- DEBUG: EXTERNAL_STAGE=/home/users/sfiligoi/dev/condor/bld_external
	-- Boost version: 1.33.1
	-- Found the following Boost libraries:
	--   thread
	CMake Error at externals/bundles/boost/1.49.0/CMakeLists.txt:110 (if):
	   if given arguments:

		 "STREQUAL" "x86_64_Fedora18"

	   Unknown arguments specified

	-- Configuring incomplete, errors occurred!
	[1528] sfiligoi@uaf-3 ~/dev/condor$ cat /etc/redhat-release
	CentOS release 5.8 (Final)
	[1529] sfiligoi@uaf-3 ~/dev/condor$ uname -a
	Linux uaf-3.t2.ucsd.edu 2.6.18-308.4.1.el5 #1 SMP Tue Apr 17 17:08:00 EDT 2012 x86_64 x86_64 x86_64 GNU/Linux

	Any suggestion on how to fix this?

	Does not make much sense to me.

	Thanks,
	   Igor