workaround -- the library renamed the pkg-config file

--- a/configure.ac
+++ b/configure.ac
@@ -37,7 +37,7 @@
 	
 	PKG_PROG_PKG_CONFIG([0.25])
 
-	PKG_CHECK_MODULES([CRYPTOPP],[libcrypto++])
+	PKG_CHECK_MODULES([CRYPTOPP],[libcryptopp])
 
 	#crypto++ headers are either in include/crypto++ or include/cryptopp,
 	#the information is otherwise unavailable from standard configuration