build-sys: provide fallback value for xsltproc

Commit ab6f56debf made the change to allow building man pages even when disabled
with ./configure --disable-manpages. This works fine, as long as xsltproc is
present. If xsltproc is not present, the command to build a man page (obviously)
fails. Unfortnately it fails with a cryptic message '-o not found', because
$(XSLTPROC) is empty. Add a fallback, to use 'xsltproc' is $(XSLTPROC) is not
defined. This way we get a nice message:

make: xsltproc: Command not found

diff --git a/Makefile.am b/Makefile.am
index 4218a6d..1f204b8 100644 (file)
--- a/Makefile.am
+++ b/Makefile.am
@@ -6043,11 +6043,12 @@ XSLTPROC_FLAGS = \
        --stringparam systemd.version $(VERSION) \
        --path '$(builddir)/man:$(srcdir)/man'
 
+XSLT = $(if $(XSLTPROC), $(XSLTPROC), xsltproc)
 XSLTPROC_PROCESS_MAN = \
-       $(AM_V_XSLT)$(XSLTPROC) -o $@ $(XSLTPROC_FLAGS) $(srcdir)/man/custom-man.xsl $<
+       $(AM_V_XSLT)$(XSLT) -o $@ $(XSLTPROC_FLAGS) $(srcdir)/man/custom-man.xsl $<
 
 XSLTPROC_PROCESS_HTML = \
-       $(AM_V_XSLT)$(XSLTPROC) -o $@ $(XSLTPROC_FLAGS) $(srcdir)/man/custom-html.xsl $<
+       $(AM_V_XSLT)$(XSLT) -o $@ $(XSLTPROC_FLAGS) $(srcdir)/man/custom-html.xsl $<
 
 man/%.1: man/%.xml man/custom-man.xsl man/custom-entities.ent
        $(XSLTPROC_PROCESS_MAN)