Add build_doc by default.

However, skip building the docs (and output warnings) if XML support is missing.
This allows `make install` to succeed w/o `make dist`.

Makefile.in

@@ -47,6 +47,7 @@ REV := @rev@
 install_suffix := @install_suffix@
 ABI := @abi@
 XSLTPROC := @XSLTPROC@
+XSLROOT := @XSLROOT@
 AUTOCONF := @AUTOCONF@
 _RPATH = @RPATH@
 RPATH = $(if $(1),$(call _RPATH,$(1)))
@@ -294,10 +295,24 @@ all: build_lib
 dist: build_doc
 
 $(objroot)doc/%.html : $(objroot)doc/%.xml $(srcroot)doc/stylesheet.xsl $(objroot)doc/html.xsl
+ifneq ($(XSLROOT),)
 	$(XSLTPROC) -o $@ $(objroot)doc/html.xsl $<
+else
+ifeq ($(wildcard $(DOCS_HTML)),)
+	@echo "<p>Missing xsltproc.  Doc not built.</p>" > $@
+endif
+	@echo "Missing xsltproc.  "$@" not (re)built."
+endif
 
 $(objroot)doc/%.3 : $(objroot)doc/%.xml $(srcroot)doc/stylesheet.xsl $(objroot)doc/manpages.xsl
+ifneq ($(XSLROOT),)
 	$(XSLTPROC) -o $@ $(objroot)doc/manpages.xsl $<
+else
+ifeq ($(wildcard $(DOCS_MAN3)),)
+	@echo "Missing xsltproc.  Doc not built." > $@
+endif
+	@echo "Missing xsltproc.  "$@" not (re)built."
+endif
 
 build_doc_html: $(DOCS_HTML)
 build_doc_man: $(DOCS_MAN3)
@@ -496,7 +511,7 @@ install_doc_man:
 	$(INSTALL) -m 644 $$d $(MANDIR)/man3; \
 done
 
-install_doc: install_doc_html install_doc_man
+install_doc: build_doc install_doc_html install_doc_man
 
 install: install_bin install_include install_lib install_doc
 

configure.ac

@@ -175,6 +175,9 @@ fi
 ],
   XSLROOT="${DEFAULT_XSLROOT}"
 )
+if test "x$XSLTPROC" = "xfalse" ; then
+  XSLROOT=""
+fi
 AC_SUBST([XSLROOT])
 
 dnl If CFLAGS isn't defined, set CFLAGS to something reasonable.  Otherwise,