Show information about flex (version and command path) in meson output

Like bison, this is useful to know which version and command of flex are
detected by meson.  For example, this was useful for me when using
win_flex.exe on Windows.

Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/ZDT3FbmrjBuBthmR@paquier.xyz

meson.build

@@ -361,6 +361,10 @@ bison_kw = {
 }
 
 flex_flags = []
+if flex.found()
+  flex_version_c = run_command(flex, '--version', check: true)
+  flex_version = flex_version_c.stdout().split(' ')[1].split('\n')[0]
+endif
 flex_wrapper = files('src/tools/pgflex')
 flex_cmd = [python, flex_wrapper,
   '--builddir', '@BUILD_ROOT@',
@@ -3370,6 +3374,7 @@ if meson.version().version_compare('>=0.57')
     {
       'bison': '@0@ @1@'.format(bison.full_path(), bison_version),
       'dtrace': dtrace,
+      'flex': '@0@ @1@'.format(flex.full_path(), flex_version),
     },
     section: 'Programs',
   )