README generation rule improved.

Author: fukuchi

Makefile.am

@@ -26,9 +26,9 @@ include_HEADERS = qrencode.h
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libqrencode.pc
 noinst_DATA = README
-CLEANFILES = README
-README: README.md
-	- ./makeREADME.sh
+README_markdown_optional := $(wildcard README.md)
+README: $(README_markdown_optional)
+	    $(if $(README_markdown_optional), ./makeREADME.sh)
 
 EXTRA_DIST = libqrencode.pc.in autogen.sh configure.ac acinclude.m4 \
 			 Makefile.am tests/Makefile.am \