[CMake] find_package to find Cygwin versions of Flex and Bison in Windows

[Stephen Morris]

To compile my application I need to use the Flex and Bison applications. To compile under Linux, all I need to do in my CMakeLists.txt file to find them is

    find_package(FLEX REQUIRED)
    find_package(BISON REQUIRED)

...and everything works as expected.

In Windows, however, I'm using the Cygwin versions of Flex and Bison which are installed in C:\cygwin64\bin. This doesn't seem to be on the path that find_package searches in Module Mode, so the search returns unsuccessfully.

However, if I try to tell find_package where to look, viz.

    find_package(FLEX REQUIRED PATHS C:/cygwin64/bin NO_DEFAULT_PATH)
    find_package(BISON REQUIRED PATHS C:/cygwin64/bin NO_DEFAULT_PATH)

...then by adding the PATHS keyword I've invoked Config mode and I get an error message saying:

    Could not find a package configuration file provided by "FLEX" with any of 
    the following names

        FLEXConfig.cmake
        flex-config.cmake

...because sure enough Cygwin does not provide these files.

So what can I do? Ideally I'd like to use Module Mode but provide a path hint; but there seems no way to do that.

At the moment I can only think of:
(a) Hard-coding add_custom_target commands to invoke flex and bison entirely manually when they're needed, or
(b) Writing my own FLEXConfig.cmake and BISONConfig.cmake files for use in Config Mode.

Both of these seem disproportionately difficult for what seems to me like it ought to be a common and simple problem. So is there a better solution that I'm missing?