Build:m4: Add AC_PROG_[FLEX/BISON] from the MESA source code.

m4/ac_prog_bison.m4

@@ -0,0 +1,18 @@
+# This script was snagged from the MESA project, which uses an MIT license.
+
+#
+# Check for Bison.
+#
+# This macro verifies that Bison is installed. If successful, then
+# 1) YACC is set to bison -y (to emulate YACC calls)
+# 2) BISON is set to bison
+#
+AC_DEFUN([AC_PROG_BISON],
+    [AC_PROG_YACC()
+    if test "$YACC" != "bison -y"; then
+        AC_SUBST(BISON,[])
+        AC_MSG_WARN([bison not found])
+    else
+        AC_SUBST(BISON,[bison])
+    fi
+])

m4/ac_prog_flex.m4

@@ -0,0 +1,18 @@
+# This script was snagged from the MESA project, which uses an MIT license.
+
+#
+# Check for FLEX.
+#
+# This macro verifies that flex is installed. If successful, then
+# 1) LEX is set to flex (to emulate lex calls)
+# 2) FLEX is set to flex
+#
+AC_DEFUN([AC_PROG_FLEX], [
+    AC_PROG_LEX(noyywrap)
+    if test "$LEX" != "flex"; then
+        AC_SUBST(FLEX,[])
+        AC_MSG_ERROR([flex not found])
+    else
+        AC_SUBST(FLEX,[flex])
+    fi
+])