libcurl: Honor $SSL_CERT_FILE

This makes libcurl search ```getenv("SSL_CERT_FILE")``` for https urls. 

I'm submitting this as a pull request as I'm not 100% sure there aren't any security implications from this. 
How about `echo '#define CURL_CA_BUNDLE getenv("CURL_CA_BUNDLE") || getenv("SSL_CERT_FILE")' >> lib/curl_config.h` instead?