build: raise minimum build requirements

 * Make gcc 7.4 and boost 1.65.1 hard dependencies, both are
   available out-of-the-box on Ubuntu 18.04
 * Require Xcode 10 on macOS but recommend 11.3 or later
 * If using clang on non-Apple platforms, the minimum supported
   version is now 6.0

Change-Id: I79c84a266741c9a66655da4fd467fe0d8218e8f0
diff --git a/docs/doxygen.conf.in b/docs/doxygen.conf.in
index 82513e4..5c857c5 100644
--- a/docs/doxygen.conf.in
+++ b/docs/doxygen.conf.in
@@ -774,7 +774,7 @@
 # spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
 # Note: If this tag is empty the current directory is searched.
 
-INPUT                  = ndn-cxx
+INPUT                  = ndn-cxx build/ndn-cxx/version.hpp
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses