make building work again when flex is not installed

This explicitly reverts commit eec7cdf
because it was a bad idea.

The motivating bug report was LudovicRousseau/PCSC#124
and the issue there occurred when building from a git clone, running
./bootstrap && ./configure && make, and having:

- configure succeed
- make "succeeeds" at having $LEX run, do nothing and fail to generate
  required sources
- compiling nonexistent files fail with highly confusing errors

The autoconf manual has always documented the correct way to handle this
is to check if lex is unavailable, and set it to the famous automake
wrapper "missing", which checks if a program is missing at build time
rather than at ./configure time, and fails the build if the rule cannot
be run. This means:

When building from a git clone, if flex is not available then
- configure succeeds
- make fails to run $LEX, and tells you to install flex

The previous attempt to fix the highly confusing error instead resulted
in configure erroring out, and saying flex is required, even when it is
*not* required because a `make dist` tarball was used, which contains
pregenerated tokenparser.c for the express purpose of making flex
unnecessary.

See autoconf documentation on $LEX:
https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Particular-Programs.html#index-AC_005fPROG_005fLEX-1

And automake documentation on why to use "missing":
https://www.gnu.org/software/automake/manual/html_node/maintainer_002dmode.html

configure.ac

@@ -33,7 +33,10 @@ AC_PROG_INSTALL
 AC_PROG_MAKE_SET
 AC_PROG_LN_S
 AC_PROG_LEX([noyywrap])
-AS_IF([test $LEX = ":"], [AC_MSG_ERROR([no lex or flex found])])
+AS_IF([test $LEX = ":"], [
+       AM_MISSING_PROG(MISSINGLEX, [flex])
+       LEX=$MISSINGLEX]
+)
 AM_PROG_AR
 PKG_PROG_PKG_CONFIG
 
@@ -331,4 +334,3 @@ AC_CONFIG_FILES(Makefile
 	examples/Makefile)
 
 AC_OUTPUT
-