Fix empy discovery for Fedora and pip

We relied on Debian and Ubuntu to provide /usr/bin/empy{,3}, but in
Fedora and pip, this fails with

    /bin/sh: empy3: command not found

Also, on Fedora and pip, empy lacks execute bits.

Use the discovered empy.

Use Python to bypass missing execute bits.

Change the error message to "empy is required".

configure.ac

@@ -15,9 +15,9 @@ AM_MAINTAINER_MODE
 PYTHON=python3
 AM_PATH_PYTHON
 
-AC_PATH_PROG([EMPY], [empy])
+AC_PATH_PROGS([EMPY], [empy empy3])
 if test -z "$EMPY"; then
-	AC_MSG_ERROR([python-empy is required])
+	AC_MSG_ERROR([empy is required])
 fi
 
 PKG_CHECK_MODULES(SHELL, gtk+-3.0)

data/Makefile.am

@@ -1,12 +1,12 @@
 SUBDIRS = icons
 
 sugar-72.gtkrc: gtkrc.em
-	empy -D scaling=\'72\' $(srcdir)/gtkrc.em > \
-		$(top_builddir)/data/sugar-72.gtkrc
+	$(PYTHON) - -D scaling=\'72\' $(srcdir)/gtkrc.em > \
+		$(top_builddir)/data/sugar-72.gtkrc < $(EMPY)
 
 sugar-100.gtkrc: gtkrc.em
-	empy -D scaling=\'100\' $(srcdir)/gtkrc.em > \
-		$(top_builddir)/data/sugar-100.gtkrc
+	$(PYTHON) - -D scaling=\'100\' $(srcdir)/gtkrc.em > \
+		$(top_builddir)/data/sugar-100.gtkrc < $(EMPY)
 
 sugardir = $(pkgdatadir)/data
 sugar_DATA =			\