Configure: options to set PolarSSL specifics.

This patch add 3 options to the configure script:

  --polarssl-headers       Path to PolarSSL headers
  --polarssl-library       Path to PolarSSL library
  --polarssl-debug-level   Set PolarSSL debug level (not implemented)

the first two aims to make things easier when using a custom or
compiled from scratch version of PolarSSL library.

Signed-off-by: Eduardo Silva <eduardo@monkey.io>