Change the CFLAGS variable assignment operator to ?= (conditional variable assignment operator).

This allows us to override CFLAGS on the command line:

$ CFLAGS='-arch $(ARCH) -gdwarf-2 -O0' ./dotest.py -C clang -A i386 -v objc-optimized

Session logs for test failures/errors will go into directory '2011-03-04-10_33_57'
Command invoked: python ./dotest.py -C clang -A i386 -v objc-optimized
----------------------------------------------------------------------
Collected 2 tests

1: test_break_with_dsym (TestObjcOptimized.ObjcOptimizedTestCase)
   Test 'expr member' continues to work for optimized build. ... ok
2: test_break_with_dwarf (TestObjcOptimized.ObjcOptimizedTestCase)
   Test 'expr member' continues to work for optimized build. ... ok

----------------------------------------------------------------------
Ran 2 tests in 1.902s

OK
$

llvm-svn: 127011