meson: Extract flex version using a regex inside a script

Different builds of Flex on different platforms output different strings in --version. For example:

macOS:

flex 2.5.35 Apple(flex-31)

Windows:

win_flex.exe 2.6.4

C:\Program Files (x86)\GnuWin32\bin\flex.EXE version 2.5.4

We need to look for a string that looks like a version, which means a regex till https://github.com/mesonbuild/meson/issues/1609 is fixed.

Fixes #356 (closed).