ext: smarter sqlcipher config params, and a user help message

[flavorjones]

Any of the standard mkmf dir_config parameters for sqlcipher now imply both --enable-system-libraries and --with-sqlcipher. Specifically these are --with-sqlicpher-dir, --with-sqlcipher-include, and --with-sqlcipher-lib.

Also I've added a --help option that emits info about the supported parameters. That help text is:

          USAGE: ruby #{$PROGRAM_NAME} [options]

            Flags that are always valid:

                --disable-system-libraries
                    Use the packaged libraries, and ignore the system libraries.
                    (This is the default behavior.)

                --enable-system-libraries
                    Use system libraries instead of building and using the packaged libraries.

                --with-sqlcipher
                    Use libsqlcipher instead of libsqlite3.
                    (Implies `--enable-system-libraries`.)

                --help
                    Display this message.


            Flags only used when using system libraries:

                General (applying to all system libraries):

                    --with-opt-dir=DIRECTORY
                        Look for headers and libraries in DIRECTORY.

                    --with-opt-lib=DIRECTORY
                        Look for libraries in DIRECTORY.

                    --with-opt-include=DIRECTORY
                        Look for headers in DIRECTORY.

                Related to sqlcipher:

                    --with-sqlcipher-dir=DIRECTORY
                        Look for sqlcipher headers and library in DIRECTORY.
                        (Implies `--with-sqlcipher` and `--enable-system-libraries`.)

                    --with-sqlcipher-lib=DIRECTORY
                        Look for sqlcipher library in DIRECTORY.
                        (Implies `--with-sqlcipher` and `--enable-system-libraries`.)

                    --with-sqlcipher-include=DIRECTORY
                        Look for sqlcipher headers in DIRECTORY.
                        (Implies `--with-sqlcipher` and `--enable-system-libraries`.)


            Flags only used when building and using the packaged libraries:

                --enable-cross-build
                    Enable cross-build mode. (You probably do not want to set this manually.)


            Environment variables used for compiling the C extension:

                CC
                    Use this path to invoke the compiler instead of `RbConfig::CONFIG['CC']`


            Environment variables passed through to the compilation of packaged libraries:

                CC
                CPPFLAGS
                CFLAGS
                LDFLAGS
                LIBS
                LT_SYS_LIBRARY_PATH
                CPP