Check for dot being available when building HTML documentation.

This helps with ensuring that the generated CHM always has the inheritance
diagrams as by default Doxygen creates it successfully even if dot is not
found -- but without any graphs.

Closes #15620.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75095 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/regen.bat

@@ -66,6 +66,23 @@ IF "%1" == "all" (
   )
 )
 
+REM Check for Graphviz (its location should be in %PATH%).
+REM
+REM NB: Always do this check because it looks like errorlevel is not set
+REM     when the test is done inside the IF block.
+dot <NUL >NUL 2>&1
+IF %GENERATE_HTML%==YES (
+    IF NOT %errorlevel%==0 (
+        IF %errorlevel%==9009 (
+            echo Error: dot was not found in PATH, please install Graphviz!
+        ) ELSE (
+            echo Error: %errorlevel% error code when running dot, please check Graphviz installation.
+        )
+
+        EXIT /B
+    )
+)
+
 REM
 REM NOW RUN DOXYGEN
 REM
@@ -74,4 +91,15 @@ REM     otherwise when generating the CHM file with Doxygen, those files are
 REM     not included!
 REM
 set PATH=%PATH%;%HHC_PATH%
-doxygen Doxyfile
+rem doxygen Doxyfile
+
+REM Check that class inheritance diagram images are present for html/chm docs.
+REM
+REM NB: Set the file to check outside the IF block,
+REM     otherwise the second check does not always pick its value.
+set filetofind=out\html\classwx_app_console__inherit__graph.png
+IF %GENERATE_HTML%==YES (
+    IF NOT EXIST %~dp0%filetofind% (
+        echo Warning: Class inheritance diagram images are missing, please check Graphviz installation.
+    )
+)