Build: Add a makefile target for Doxygen documentation

You can now build the doxygen documentation with `make docs` and clean it with `make clean-docs`.

Fixes: #11949

Makefile.am

@@ -277,6 +277,22 @@ CLEANFILES = $(OSX_DMG) $(BITCOIN_WIN_INSTALLER)
 
 DISTCHECK_CONFIGURE_FLAGS = --enable-man
 
-clean-local:
+doc/doxygen/.stamp: doc/Doxyfile FORCE
+	$(MKDIR_P) $(@D)
+	$(DOXYGEN) $^
+	$(AM_V_at) touch $@
+
+if HAVE_DOXYGEN
+docs: doc/doxygen/.stamp
+else
+docs:
+	@echo "error: doxygen not found"
+endif
+
+clean-docs:
+	rm -rf doc/doxygen
+
+clean-local: clean-docs
 	rm -rf coverage_percent.txt test_bitcoin.coverage/ total.coverage/ test/tmp/ cache/ $(OSX_APP)
 	rm -rf test/functional/__pycache__ test/functional/test_framework/__pycache__ test/cache
+

configure.ac

@@ -93,6 +93,11 @@ AC_PATH_PROG(HEXDUMP,hexdump)
 AC_PATH_TOOL(READELF, readelf)
 AC_PATH_TOOL(CPPFILT, c++filt)
 AC_PATH_TOOL(OBJCOPY, objcopy)
+AC_PATH_PROG(DOXYGEN, doxygen)
+if test -z "$DOXYGEN"; then
+   AC_MSG_WARN([Doxygen not found])
+fi
+AM_CONDITIONAL([HAVE_DOXYGEN], [test -n "$DOXYGEN"])
 
 AC_ARG_VAR(PYTHONPATH, Augments the default search path for python module files)
 
@@ -1257,7 +1262,7 @@ AC_SUBST(PROTOBUF_LIBS)
 AC_SUBST(QR_LIBS)
 AC_CONFIG_FILES([Makefile src/Makefile doc/man/Makefile share/setup.nsi share/qt/Info.plist test/config.ini])
 AC_CONFIG_FILES([contrib/devtools/split-debug.sh],[chmod +x contrib/devtools/split-debug.sh])
-AC_CONFIG_FILES([doc/Doxyfile])
+AM_COND_IF([HAVE_DOXYGEN], [AC_CONFIG_FILES([doc/Doxyfile])])
 AC_CONFIG_LINKS([contrib/filter-lcov.py:contrib/filter-lcov.py])
 AC_CONFIG_LINKS([test/functional/test_runner.py:test/functional/test_runner.py])
 AC_CONFIG_LINKS([test/util/bitcoin-util-test.py:test/util/bitcoin-util-test.py])

doc/developer-notes.md

@@ -132,6 +132,8 @@ Not OK (used plenty in the current source, but not picked up):
 A full list of comment syntaxes picked up by doxygen can be found at http://www.stack.nl/~dimitri/doxygen/manual/docblocks.html,
 but if possible use one of the above styles.
 
+Documentation can be generated with `make docs` and cleaned up with `make clean-docs`.
+
 Development tips and tricks
 ---------------------------