Non-obvious build failure when lex not installed

[tsavola]

configure succeeds even when there is no lex installed. However, it does mention it:

...
checking for flex... no
checking for lex... no
...

After the successful configure, make fails like this:

  CC       src/main.o
  LEX      src/libeasyseccomp_a-lexer.c
  CC       src/libeasyseccomp_a-libeasyseccomp_a-lexer.o
gcc: error: ./src/libeasyseccomp_a-lexer.c: No such file or directory
gcc: fatal error: no input files
compilation terminated.
make: *** [Makefile:765: src/libeasyseccomp_a-libeasyseccomp_a-lexer.o] Error 1

It took some figuring out to learn that I had to install flex. (I'm using Ubuntu 20.04.)

[giuseppe]

thanks for opening the issue.

Indeed it is annoying, I've pushed a commit to check for lex an yacc presence, otherwise fail