remove spurious flex dependency

nothing in a normal gcc/binutils actually uses flex and bison, but
this useless test for the output filename lex produces breaks the
build unless [f]lex is installed. the result of this check is
specified by posix anyway, so hard-code the specified result.

litecross/Makefile

@@ -16,6 +16,7 @@ MUSL_CONFIG = CC="$(XGCC)" LIBCC="../obj_toolchain/$(TARGET)/libgcc/libgcc.a" --
 -include config.mak
 
 MAKE += INFO_DEPS=
+MAKE += ac_cv_prog_lex_root=lex.yy.c
 
 FULL_TOOLCHAIN_CONFIG = $(TOOLCHAIN_CONFIG) \
 	--disable-werror \