doc/api: fix the doxygen output handling

Using output: ['.'] broke ninja after ninja clean - it removed the whole
directory and thus the meson-generated configure_files (i.e. all the
doxygen sources we copied). ninja didn't know how to build those.

Fix this by rearranging the doxygen output to build into a different
directory now and setting the output to that. This doesn't exactly *fix*
things since that directory is no longer removed during ninja clean, but at
least the build no longer fails.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

doc/api/libinput.doxygen.in

@@ -11,8 +11,9 @@ WARNINGS               = YES
 QUIET                  = YES
 INPUT                  = "@builddir@"
 IMAGE_PATH             = "@builddir@"
+OUTPUT_DIRECTORY       = doc
 GENERATE_HTML          = YES
-HTML_OUTPUT            = api
+HTML_OUTPUT            = html
 SEARCHENGINE           = NO
 USE_MATHJAX            = YES
 MATHJAX_RELPATH        = https://cdn.mathjax.org/mathjax/latest

doc/api/meson.build

@@ -45,7 +45,7 @@ doxyfile = configure_file(input : 'libinput.doxygen.in',
 
 custom_target('doxygen',
 	      input : [ doxyfiles, doxyfile, mainpage ] + src_doxygen,
-	      output : [ '.' ],
+	      output : [ 'html' ],
 	      command : [ doxygen, doxyfile ],
 	      install : false,
 	      depends: [ mainpage ],