Seek lib deps via CMake FetchContent

`FetchContent` respects existing installs, and downloads and compiles the libs
if they aren't found.
This is admittedly a little finicky, since this ignores the usual `Find*`
modules provided by CMake, requiring a bit of glue on our side.

But, one upside is that this moves that logic from our CI into the build system,
which can thus benefit other downstream users.
This also opens the door to some improvements in upcoming commits.

Doing this with Bison turned out to be much more painful, however, due to
`FindBISON` providing the specific `bison_target` command; thus, it remains
installed externally, so that it can be picked up by `FindBISON`.

This also bumps our CMake version requirement slightly, though it's
possible that older versions keep working, or could be supported with small patches;
however, our CI doesn't provide anything below 3.31, so we can't check.

src/CMakeLists.txt

@@ -13,7 +13,7 @@ target_compile_definitions(common PRIVATE "BUILD_VERSION_STRING=\"${GIT_REV}\"")
 
 find_package(BISON 3.0.0 REQUIRED)
 set(BISON_FLAGS "-Wall -Dlr.type=ielr")
-# Set some flags on versions that support them
+# Set some flags on versions that support them.
 if(BISON_VERSION VERSION_GREATER_EQUAL "3.5")
   set(BISON_FLAGS "${BISON_FLAGS} -Dparse.lac=full -Dapi.token.raw=true -Wdangling-alias")
 endif()