configure: Use `AM_MISSING_PROG' for Bison.

* configure.ac: Use `AM_MISSING_PROG' instead of `AC_PROG_YACC'.

* gcc-plugin/src/Makefile.am (AM_YFLAGS): New variable.

configure.ac

@@ -1231,7 +1231,7 @@ fi
 
 # Bison is used to generate the C expression parser.  The generated
 # parser is part of the distribution, though.
-AC_PROG_YACC
+AM_MISSING_PROG([YACC], [bison])
 
 AM_CONDITIONAL([BUILD_GCC_PLUGIN], [test "x$build_gcc_plugin" = "xyes"])
 AM_CONDITIONAL([HAVE_GUILE], [test "x$GUILE" != "x"])

gcc-plugin/src/Makefile.am

@@ -19,6 +19,10 @@ gccplugin_LTLIBRARIES = starpu.la
 
 starpu_la_SOURCES = starpu.c c-expr.y
 
+# Use the Yacc-compatibility mode so that Bison doesn't error out upon
+# reduce/reduce conflicts.
+AM_YFLAGS = -y
+
 AM_CPPFLAGS =						\
   -I$(top_srcdir)/include				\
   -I$(GCC_PLUGIN_INCLUDE_DIR) -Wall -DYYERROR_VERBOSE=1