build: Enable gtk-doc during distcheck

gtk-doc works just fine with srcdir != builddir.

NEWS

+Overview of changes in 3.19.92.1
+================================
+
+* Bugs fixed:
+ 763078 Documentation missing from tarball
+
+
 Overview of changes in 3.19.92
 ==============================
 

configure.ac

 m4_define([goa_major_version], [3])
 m4_define([goa_minor_version], [19])
 m4_define([goa_micro_version], [92])
-m4_define([goa_nano_version], [0])
+m4_define([goa_nano_version], [1])
 
 m4_define([goa_base_version],
           [goa_major_version.goa_minor_version.goa_micro_version])