[SCons] Install docs based on prior doc build

Also eliminate extra 'html' subdirectory in install destination

SConstruct

@@ -1994,7 +1994,7 @@ if env['CC'] != 'cl':
     VariantDir('build/platform', 'platform/posix', duplicate=0)
     SConscript('build/platform/SConscript')
 
-if env['doxygen_docs'] or env['sphinx_docs']:
+if env['doxygen_docs'] or env['sphinx_docs'] or "install" in COMMAND_LINE_TARGETS:
     SConscript('doc/SConscript')
 
 # Sample programs (also used from test_problems/SConscript)
@@ -2067,7 +2067,7 @@ def postInstallMessage(target, source, env):
             name=name, location=env_dict[location]
         ))
 
-    if env["sphinx_docs"] or env["doxygen_docs"]:
+    if env["installed_docs"]:
         name = "HTML documentation"
         install_message.append(locations_message.format(
             name="HTML documentation", location=env_dict["inst_docdir"]

doc/SConscript

@@ -133,5 +133,8 @@ if localenv['sphinx_docs']:
     localenv.AlwaysBuild(sphinxdocs)
     if localenv['doxygen_docs']:
         localenv.Depends(sphinxdocs, docs)
-    install(localenv.RecursiveInstall, '$inst_docdir/html',
-            '#build/doc/html', exclude=['\\.map', '\\.md5'])
+
+doc_files = install(localenv.RecursiveInstall, '$inst_docdir',
+                    '#build/doc/html', exclude=['\\.map', '\\.md5'])
+
+env['installed_docs'] = bool(doc_files)