Explicitly set the openssl include directory

[rmccrystal]

I am trying to compile this crate on Windows with openssl using YARA_CRYPTO_LIB=openssl to fix the issue described by #89 and it complains that it cannot find an openssl header file. I fixed the issue on my end by adding a manual call to cc::Build::include inside build.rs to my openssl include directory, but it doesn't seem like there is any way to do this with environment variables

        let mut cc = cc::Build::new();
        cc.include(&basedir)
            .include(basedir.join("include"))
            .include(basedir.join("modules"))
            .include("C:\\Program Files\\OpenSSL-Win64\\include");

[rmccrystal]

98 fixes this

[ikrivosheev]

@rmccrystal can we close the issue?