Build: Add --enable-doxygen to generate and install API docs

It requires Doxygen. This option is disabled by default.

(cherry picked from commit e21efdf96f)

INSTALL

@@ -352,6 +352,12 @@ XZ Utils Installation
                 will still be installed. The $docdir can be changed
                 with --docdir=DIR.
 
+    --enable-doxygen
+                Enable generation of the HTML version of the liblzma API
+                documentation using Doxygen. The resulting files are
+                installed to $docdir/api. This option assumes that
+                the 'doxygen' tool is available.
+
     --disable-assembler
                 This disables CRC32 and CRC64 assembly code on
                 32-bit x86. This option currently does nothing

configure.ac

@@ -516,10 +516,18 @@ AM_CONDITIONAL([COND_SCRIPTS], [test x$enable_scripts != xno])
 
 AC_ARG_ENABLE([doc], [AS_HELP_STRING([--disable-doc],
 		[do not install documentation files to docdir
-		(man pages will still be installed)])],
+		(man pages are still installed and,
+		if --enable-doxygen is used,
+		liblzma API documentation is installed too)])],
 	[], [enable_doc=yes])
 AM_CONDITIONAL([COND_DOC], [test x$enable_doc != xno])
 
+AC_ARG_ENABLE([doxygen], [AS_HELP_STRING([--enable-doxygen],
+		[generate HTML version of the liblzma API documentation
+		using Doxygen and install the result to docdir])],
+	[], [enable_doxygen=no])
+AM_CONDITIONAL([COND_DOXYGEN], [test x$enable_doxygen != xno])
+
 
 ##############
 # Sandboxing #

src/liblzma/api/Makefile.am

@@ -17,3 +17,22 @@ nobase_include_HEADERS = \
 	lzma/stream_flags.h \
 	lzma/version.h \
 	lzma/vli.h
+
+if COND_DOXYGEN
+$(top_builddir)/doc/api/index.html: $(top_srcdir)/doxygen/update-doxygen $(top_srcdir)/doxygen/Doxyfile $(nobase_include_HEADERS)
+	$(MKDIR_P) "$(top_builddir)/doc"
+	"$(top_srcdir)/doxygen/update-doxygen" api \
+		"$(top_srcdir)" "$(top_builddir)/doc"
+
+all-local: $(top_builddir)/doc/api/index.html
+
+install-data-local:
+	$(MKDIR_P) "$(DESTDIR)$(docdir)/api"
+	$(INSTALL_DATA) "$(top_builddir)"/doc/api/* "$(DESTDIR)$(docdir)/api"
+
+uninstall-local:
+	rm -rf "$(DESTDIR)$(docdir)/api"
+
+clean-local:
+	rm -rf "$(top_builddir)/doc/api"
+endif