If you want to find the libmcrypt library in other projects using autoconf/aclocal, put in your aclocal.m4: include(libmcrypt.m4) configure.in: AM_PATH_LIBMCRYPT( 2.4.11,, AC_MSG_ERROR([[*** libmcrypt was not found]])) these macros define LIBMCRYPT_LIBS and LIBMCRYPT_CFLAGS. If Libmcrypt version 2.2.x is found then LIBMCRYPT22 is defined, otherwise LIBMCRYPT24.