Update Makefile.am

Allows setting the path for ronn and kramdown

Makefile.am

 SHARED_VERSION=2:51:1 # = *.so.1.1.51
 
 MKDIR=mkdir -p
+RONN=ronn
+KRAMDOWN=kramdown
 
 # -Wno-endif-labels : Needed to prevent warnings in ieee80.c.
 AM_CFLAGS = \
 .md.html: _layouts/webpage.html
 	@echo "  MD        $@"
 	@cat $< | sed -e 's/\.md)/.html)/g' -e 's/\.ronn/.html/g' | \
-		kramdown --template _layouts/webpage.html > $@
+		$(KRAMDOWN) --template _layouts/webpage.html > $@
 
 .ronn.html:
-	ronn --html $<
+	$(RONN) --html $<
 
 ##### vim:
 
 ##### documentation:
 
 src/espeak-ng.1: src/espeak-ng.1.ronn
-	ronn --roff $<
+	$(RONN) --roff $<
 
 src/speak-ng.1: src/speak-ng.1.ronn
-	ronn --roff $<
+	$(RONN) --roff $<
 
 docs_MARKDOWN != ls docs/*.md docs/*/*.md docs/*/*/*.md
 docs_HTML = ${docs_MARKDOWN:.md=.html}