Add m4 macros for checking for a perl module.

I just encountered same problem as #29 and this patch is supposed to fix the following problem:

```bash
$ uname -a
Linux osvm0839 2.6.32-431.1.2.el6.x86_64 #1 SMP Thu Dec 12 13:59:19 CST 2013 x86_64 x86_64 x86_64 GNU/Linux

$ ./autogen.sh && ./configure && make
[...]
make[3]: Leaving directory `/root/Sources/heimdal.git/lib/wind'
make[2]: Leaving directory `/root/Sources/heimdal.git/lib/wind'
Making all in asn1
make[2]: Entering directory `/root/Sources/heimdal.git/lib/asn1'
cd . && perl ../../cf/make-proto.pl -q -P comment -o der-protos.h der_locl.h der.c der.h der_get.c der_put.c der_free.c der_length.c der_copy.c der_cmp.c der_format.c fuzzer.c heim_asn1.h extra.c roken_rename.h template.c timegm.c || rm -f der-protos.h
Can't locate JSON.pm in @INC (@INC contains: /usr/local/lib64/perl5 /usr/local/share/perl5 /usr/lib64/perl5/vendor_perl /usr/share/perl5/vendor_perl /usr/lib64/perl5 /usr/share/perl5 .) at ../../cf/make-proto.pl line 7.
BEGIN failed--compilation aborted at ../../cf/make-proto.pl line 7.
cd . && perl ../../cf/make-proto.pl -q -P comment -p der-private.h der_locl.h der.c der.h der_get.c der_put.c der_free.c der_length.c der_copy.c der_cmp.c der_format.c fuzzer.c heim_asn1.h extra.c roken_rename.h template.c timegm.c || rm -f der-private.h
Can't locate JSON.pm in @INC (@INC contains: /usr/local/lib64/perl5 /usr/local/share/perl5 /usr/lib64/perl5/vendor_perl /usr/share/perl5/vendor_perl /usr/lib64/perl5 /usr/share/perl5 .) at ../../cf/make-proto.pl line 7.
BEGIN failed--compilation aborted at ../../cf/make-proto.pl line 7.
compile_et asn1_err.et
  CC     asn1parse.o
In file included from gen_locl.h:54,
                 from asn1parse.y:48:
der.h:101:24: error: der-protos.h: No such file or directory
In file included from asn1parse.y:48:
gen_locl.h:55:25: error: der-private.h: No such file or directory
make[2]: *** [asn1parse.o] Error 1
make[2]: Leaving directory `/root/Sources/heimdal.git/lib/asn1'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/root/Sources/heimdal.git/lib'
make: *** [all-recursive] Error 1
```

The `configure` only shows out a warning when perl module are missing.

```bash
$ ./configure
[...]
checking for perl... perl
configure: WARNING: perl module JSON not found - some stuff will not build
[...]
```