llvm3.3: fix error output from configure if CFLAGS is set

Error is cosmetic, but distracting. Backport upstream fix.

  | .../build-vbox32/tmp/work/core2-32-rdk-linux-musl/llvm3.3/3.3-r0/llvm-3.3.src/configure: line 1990: -O2: command not found
  | .../build-vbox32/tmp/work/core2-32-rdk-linux-musl/llvm3.3/3.3-r0/llvm-3.3.src/configure: line 1991: -O2: command not found

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>