not just CGO_CFLAGS_ALLOW='-mfoo'. Similarly named variables control
the allowed CPPFLAGS, CXXFLAGS, FFLAGS, and LDFLAGS.

Also for security reasons, only a limited set of characters are
permitted, notably alphanumeric characters and a few symbols, such as
'.', that will not be interpreted in unexpected ways. Attempts to use
forbidden characters will get a "malformed #cgo argument" error.