package/exim: add libxcrypt optional dependency

Fix the following build failure raised since bump of glibc to version
2.39 in commit b5680f53d60acf8ff6010082f873438a39bd5d97:

crypt16.c:45:10: fatal error: crypt.h: No such file or directory
   45 | #include <crypt.h>
      |          ^~~~~~~~~

Fixes: b5680f53d60acf8ff6010082f873438a39bd5d97
 - http://autobuild.buildroot.org/results/3c846c763a05f2569db7a019af8b91b0563c8f42

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/exim/Config.in

@@ -7,6 +7,7 @@ config BR2_PACKAGE_EXIM
 	depends on BR2_TOOLCHAIN_HAS_THREADS
 	select BR2_PACKAGE_PCRE2
 	select BR2_PACKAGE_BERKELEYDB
+	select BR2_PACKAGE_LIBXCRYPT if BR2_TOOLCHAIN_USES_GLIBC
 	help
 	  Exim is a message transfer agent (MTA) developed at the
 	  University of Cambridge for use on Unix systems connected to

package/exim/exim.mk

@@ -75,6 +75,10 @@ define EXIM_USE_DEFAULT_CONFIG_FILE_CLAMAV
 endef
 endif
 
+ifeq ($(BR2_PACKAGE_LIBXCRYPT),y)
+EXIM_DEPENDENCIES += libxcrypt
+endif
+
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
 EXIM_DEPENDENCIES += host-openssl openssl
 define EXIM_USE_DEFAULT_CONFIG_FILE_OPENSSL