[misc] Search for the crypt symbol to conditionally enable crypt in VTY

It will define -DVTY_CRYPT_PW= if crypt is found. If -lcrypt is
needed, $(LIB_CRYPT) will be -lcrypt.

Compiles on OS X and Linux and reenables crypt.

Patch by: Lars Immisch
diff --git a/src/Makefile.am b/src/Makefile.am
index afa63cc..7fd7049 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -14,7 +14,7 @@
 libvty_a_SOURCES = vty/buffer.c vty/command.c vty/vector.c vty/vty.c
 
 bsc_hack_SOURCES = bsc_hack.c vty_interface.c
-bsc_hack_LDADD = libbsc.a libvty.a -ldl -ldbi
+bsc_hack_LDADD = libbsc.a libvty.a -ldl -ldbi $(LIBCRYPT)
 
 bs11_config_SOURCES = bs11_config.c abis_nm.c gsm_data.c msgb.c debug.c \
 		      select.c timer.c rs232.c tlv_parser.c signal.c
@@ -22,4 +22,4 @@
 ipaccess_find_SOURCES = ipaccess-find.c select.c timer.c
 
 ipaccess_config_SOURCES = ipaccess-config.c
-ipaccess_config_LDADD = libbsc.a libvty.a -ldl -ldbi
+ipaccess_config_LDADD = libbsc.a libvty.a -ldl -ldbi $(LIBCRYPT)