build: fail if bison/flex are not available

Problem: if flex or bison are not installed, configure
lets the build continue and fail.

Ensure that if those programs are missing, configure fails.

Fixes #40

configure.ac

@@ -45,7 +45,11 @@ AC_SUBST([WARNING_CFLAGS])
 AC_PROG_LN_S
 AC_PROG_MAKE_SET
 AC_PROG_LEX([noyywrap])
+AS_IF([test "x$LEX" = "x:"],AC_MSG_ERROR([could not find flex]),[])
 AC_PROG_YACC
+# AC_PROG_YACC sets YACC=yacc when nothing is found, even yacc
+# Just assume bison is the preferred compiler-compiler and call yacc a failure
+AS_IF([test "x$YACC" = "xyacc"],AC_MSG_ERROR([could not find bison]),[])
 AM_CONDITIONAL(WITH_GNU_LD, test "$with_gnu_ld" = "yes")
 
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