Add a check for openssl that accepts explicit file locations.

m4/openssl.m4

@@ -0,0 +1,54 @@
+dnl Check to find the OpenSSL headers/libraries
+
+
+AC_DEFUN(tinc_OPENSSL,
+[
+
+  AC_CACHE_CHECK([for openssl], tinc_cv_openssl_include,
+  [
+    AC_ARG_WITH(openssl,
+      [  --with-openssl=DIR      OpenSSL library and headers prefix],
+      [echo -ne 'prefix...'
+       openssl_lib="$withval/lib"
+       openssl_include="$withval/include"]
+    )
+    AC_ARG_WITH(openssl-include,
+      [  --with-openssl-include=DIR OpenSSL headers directory],
+      [echo -ne 'includes...'
+       openssl_include="$withvall"]
+    )
+    AC_ARG_WITH(openssl-lib,
+      [  --with-openssl-lib=DIR  OpenSSL library directory],
+      [echo -ne 'libraries...'
+       openssl_lib="$withval"]
+    )
+
+if test "x$openssl_lib" != "x" ; then
+  LIBS="$LIBS -L$openssl_lib"
+fi
+if test "x$openssl_include" != "x" ; then
+  INCLUDES="$INCLUDES -I$openssl_include"
+fi
+
+AC_MSG_RESULT([done.])
+  ])
+
+libcrypto=none
+AC_CHECK_LIB(crypto, SHA1_version, [
+  libcrypto=yes
+])
+
+if test $libcrypto = none; then
+  AC_MSG_ERROR(
+[It seems like OpenSSL is not installed on this system.  But perhaps
+you need to supply the exact location of the headers and libraries.
+You can try running configure with the --with-openssl=/DIRECTORY
+parameter.  If you installed the headers and libraries in a different
+location you can use --with-openssl-include=/DIR and
+--with-openssl-lib=/DIR.])
+
+else
+  LIBS="$LIBS -lcrypto"
+fi
+
+])