Add configure option to not build libfl (Feature request)

[Explorer09]

Copied from SourceForge bug report #200 (Reported my me also)

Building flex without libfl is useful when you know that this flex build of yours will be used only with a cross-toolchain. For cross-building environments, having a libfl built for the host architecture is not useful. (We need one for the target architecture instead.)

For my case, although I would need to configure and build flex twice for a cross environment, I need to use a hack to disable libfl currently:

sed -i.bak -e 's/lib_LTLIBRARIES = \$/lib_LTLIBRARIES =/' \
    -e 's/\tlibfl.la \$/#&/' -e 's/\tlibfl_pic.la$/#&/' src/Makefile.in

It's better to make it into a configure option. (For your reference, GNU Bison can disable building liby by the '--disable-yacc' option.)

Alternatively, add a configure-time variable to explicitly specify which compiler will be used to build libfl (could be named CC_FOR_TARGET or CC_FOR_LIBFL). If using this approach, don't forget the respective CFLAGS_FOR_TARGET and other variables.