cmake: require bison >= 2.5 and flex

[Andrej730]

Hi! Adding bison requirement to cmake to catch errors like #27 early, during configuration.

Either way, bison is already required, building without it breaks at configuration stage:

CMake Error at CMakeLists.txt:48 (bison_target):
  Unknown CMake command "bison_target".

With 2.5 REQUIRED it now will result in:

  Could NOT find BISON (missing: BISON_EXECUTABLE) (Required is at least
  version "2.5")

Or if bison is found (important on Mac, which is stuck on 2.3 and would break during the build with more obscure invalid option -- W):

  Could NOT find BISON: Found unsuitable version "2.3", but required is at
  least "2.5"

Why 2.5:

• 2.3 - breaks with invalid option -- W
• 2.4 - breaks with invalid argument no-conflicts-rr for --warnings
• 2.5 - (at least) builds fine

@chronoxor can you please take a look?

[chronoxor]

Did you check this MacOS build in github, should work out of the box
https://github.com/chronoxor/FastBinaryEncoding/actions/runs/34533534842/job/103059696950

-- The System identification is Darwin 25.6.0 Platform/Darwin
-- Found BISON: /opt/homebrew/opt/bison/bin/bison (found version "3.8.2")
-- Found FLEX: /opt/homebrew/opt/flex/bin/flex (found version "2.6.4")

[Andrej730]

Did you check this MacOS build in github, should work out of the box
chronoxor/FastBinaryEncoding/actions/runs/34533534842/job/103059696950

It works, because CI prefixes PATH. If user doesn't install bison from brew / install it without prefixing PATH, it will fallback to default Mac bison 2.3

FastBinaryEncoding/.github/workflows/build-macos.yml

Lines 19 to 22 in 7347014

	- name: "Setup flex, bison"
	run: |
	brew install flex bison
	echo "/opt/homebrew/opt/flex/bin:/opt/homebrew/opt/bison/bin" >> "$GITHUB_PATH"

Here's an example failing build using this commit e37a47f - https://github.com/Andrej730/FastBinaryEncoding/actions/runs/36319928061/job/108621678066

-- Found BISON: /usr/bin/bison (found version "2.3")
...
 /Applications/Xcode_26.6.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/bison: invalid option -- W
Try `/Applications/Xcode_26.6.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/bison --help' for more information.
make[2]: *** [/Users/runner/work/FastBinaryEncoding/FastBinaryEncoding/source/fbe-parser.cpp] Error 1
make[2]: *** Waiting for unfinished jobs....

[Andrej730]

Since flex is also required, I've added similar change for flex, for consistency. So it will error now:

CMake Error at /usr/share/cmake-3.28/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find FLEX (missing: FLEX_EXECUTABLE)

instead of

CMake Error at CMakeLists.txt:46 (flex_target):
  Unknown CMake command "flex_target"

In case flex is missing.

Unsure if there's any particular version requirement for flex, so the version is omitted.