[README,doc,meson*] Rework documentation build

* Avoid going through hoops to silently enable docs if doxygen is
  available
* Require build-doc explicitly set to true before scanning for doxygen
* build-doc is conservatively false by default
* Update README

README.md

@@ -90,15 +90,18 @@ for more advanced build management tasks.
 The Micro BGP Suite uses [Doxygen](https://www.doxygen.org/index.html) to document its API.
 
 In order to build `µbgpsuite` documentation you must enable the `build-doc`
-configure flag. This flag is enabled automatically if you have `doxygen`
-installed in your system when you run `meson` to configure the project for the
-first time.
+configure flag.
+
+To do so, run the following command inside an already existing `build` directory:
+```sh
+$ meson configure -Dbuild-doc=true
+```
 
 Once the flag is enabled, you can use:
 ```sh
 $ ninja doc
 ```
-inside build directory to generate Doxygen documentation.
+inside the build directory to generate the Doxygen documentation.
 
 You can access the documentation by pointing your web browser to
 `doc/html/index.html` inside the build directory.

doc/meson.build

@@ -6,6 +6,8 @@ cdata = configuration_data({
     'PROJECT_NAME': meson.project_name(),
 })
 
+doxygen = find_program('doxygen')
+
 doxyfile = configure_file(input : 'Doxyfile.in',
                           output : 'Doxyfile',
                           configuration : cdata,

meson.build

@@ -97,11 +97,8 @@ lonetix_dep = declare_dependency(compile_args : lonetix_args,
 # TODO
 # endif
 
-doxygen = find_program('doxygen', required : get_option('build-doc'))
-if doxygen.found()
+if get_option('build-doc')
     subdir('doc')
-elif get_option('build-doc').auto()
-    message('Not building documentation as doxygen was not found')
 endif
 
 if get_option('build-bgpgrep')

meson_options.txt

@@ -1,5 +1,5 @@
 option('native', type : 'boolean', value : false, description : 'Optimize build for local CPU architecture (ties build artifacts to your local machine!)')
-option('build-doc', type : 'boolean', description : 'Build documentation using doxygen')
+option('build-doc', type : 'boolean', value : false, description : 'Build documentation using doxygen')
 # TODO option('build-benchmarks', type : 'boolean', value : false, description : 'Build benchmarks')
 option('build-tests', type : 'boolean', value : false, description : 'Build tests')
 option('build-bgpgrep', type : 'boolean', description : 'Build bgpgrep MRT/BGP parsing and filting utility')