re2 previously failed to compile if:

1. An old `re2` version is installed with a non-standard system
prefix, such as `/opt/local`.
2. The environment variable is set: `CPPFLAGS=-I/opt/local/include`.

Running `make` would result in function prototype mismatches because
the Makefile would previously attempt to use the headers from
`/opt/local/include/re2` before the `third_party/re2/re2`
directory.

To ensure the current directories receive priority, include the
relevant re2 directories in the include path.

This reverts commit 37a6ff0.

grpc#27660 caused `CPPFLAGS` to inherit
from the environment, but this can cause the Makefile to use external
include files for re2 and other libraries if `-I` flags are defined.

This commit reverts to the original behavior of only using
`RbConfig::CONFIG` values to avoid using the wrong headers.

This makes it possible to override the compiler and other programs.