Getting a CMake Error on FLEX_INCLUDE_DIR about nyan

[cmlccie]

Manually setting a FLEX_INCLUDE_DIR to the path to the flex include directory doesn't change the error.

[tusharpm]

You can try to set FLEX_INCLUDE_DIRS manually, in case CMake failed to find it automatically.
FLEX_INCLUDE_DIR is internal to CMake. FindFLEX module defines FLEX_INCLUDE_DIRS as the external variable.

[nyx]

@cmlccie I ran into this as well on macOS literally yesterday. I had installed flex v2.6.4 (which was another gotcha, the system version is not new enough and the generated code uses deprecated register qualifier for C++17 standard. Anyway after I brew install flex, I ended up adding that to the the head of my PATH environment variable as Homebrew suggested so it's supercede the system one as explicitly setting it in CMake apparently isn't respected everywhere, I then did brew info again and it gives you the LDFLAGS and CPPFLAGS to use for builds. I grabbed those and threw them into the CMake configuration.

 FLEX_EXECUTABLE                  /usr/local/opt/flex/bin/flex                                                                             
 FLEX_INCLUDE_DIR                 /usr/local/opt/flex/include                                                                              
 FL_LIBRARY                       /usr/local/opt/flex/lib/libfl.dylib 

[tusharpm]

Yes, the system version is flex 2.5.35 Apple(flex-31) on macOS 10.13.2 High Sierra. The change removing register is in release 2.6.0, which can be called the minimum required flex version for the project.
Specifying the FLEX_EXECUTABLE and FLEX_INCLUDE_DIR on the cmake command line suffices. The PATH can be left unmodified. It works without setting FL_LIBRARY for now; however, it might be a good idea to set that also - in case we add it as nyan's link dependency in the future.
I got a linker error afterwards for using -Wl,--no-undefined.
I've captured the relevant changes in #25. Please have a look at the PR and provide your review comments.

[TheJJ]

Please report back if you still encounter problems.