introduce library version plus git hash

This exposes the library version and git head hash it was built from
into LWS_LIBRARY_VERSION and LWS_BUILD_HASH.

These are combined into a version string that's both printed as a
notice log by the library and made available to the app using a new
api lws_get_library_version().  The version looks like

 1.1 178d78c

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/libwebsockets-api-doc.html b/libwebsockets-api-doc.html
index 46fe49a..e6b424f 100644
--- a/libwebsockets-api-doc.html
+++ b/libwebsockets-api-doc.html
@@ -85,6 +85,23 @@
 This function creates a connection to a remote server
 </blockquote>
 <hr>
+<h2>lws_get_library_version - </h2>
+<i>const char *</i>
+<b>lws_get_library_version</b>
+(<i></i> <b>void</b>)
+<h3>Arguments</h3>
+<dl>
+<dt><b>void</b>
+<dd>no arguments
+</dl>
+<h3>Description</h3>
+<blockquote>
+<p>
+returns a const char * to a string like "1.1 178d78c"
+representing the library version followed by the git head hash it
+was built from
+</blockquote>
+<hr>
 <h2>libwebsockets_hangup_on_client - Server calls to terminate client connection</h2>
 <i>void</i>
 <b>libwebsockets_hangup_on_client</b>