Check envvars of `CPPFLAGS` and `LDFLAGS` during tests
Don't clober the environment `CPPFLAGS` and `LDFLAGS` (refs #168)

Because the variables specified via command-line arguments for the
`./configure` will be favored than one in environment variables,
setting those variables in `PACKAGE_CONFIGURE_OPTS_ARRAY` will hide
existing environment variables.

To avoid the problem, stop using `package_option()` to setup those
variables.
Merge branch 'dont_clober_ldflags_cppflags' (fixes #168)