Make auto-reconfiguring work if CFLAGS contains more than one flag.

When using an open-source clang on OS X, one has to pass an isysroot
flag so that it can find system headers (stdio.h), like so:

  CXX=path/to/clang++ CFLAGS="-isysroot $(xcrun -show-sdk-path)" ./configure.py

Previously, configure.py wouldn't quote envvars containing spaces, so
it'd rerun this as

  CXX=path/to/clang++ CFLAGS=-isysroot /sysroot/path ./configure.py

which would then die because /sysroot/path wasn't excecutable.