build-sys: properly quote m4 macro arguments in CC_CHECK_FLAGS_APPEND

The variables should be quoted inside [...] to avoid double macro
expansion.  This is currently not an issue, since the values (-W...) are
not really macros, but we might as well just fix that issue now.

Tested by re-running autogen.sh and comparing the value of OUR_CFLAGS in
the generated Makefile.  Ran a full build from a clean tree to confirm
no other issues were introduced.