Fix cross-compiling when 128-bit math compiles but won't link

Apparently, there exist cross-compiling environments for arm7 where
you can compile a 64x64->128 multiply, but not link it.

Fixes bug 11729; bugfix on 0.2.4.8-alpha. Patch from 'conradev'.

changes/bug11729

@@ -0,0 +1,5 @@
+  o Minor bugfixes (build):
+    - When deciding whether to build the 64-bit curve25519 implementation,
+      detect platforms where we can compile 128-bit arithmetic but cannot
+      link it. Fixes bug 11729; bugfix on 0.2.4.8-alpha. Patch
+      from "conradev".

configure.ac

@@ -770,7 +770,7 @@ if test x$enable_curve25519 != xno; then
         ])],
 	[tor_cv_can_use_curve25519_donna_c64=yes],
         [tor_cv_can_use_curve25519_donna_c64=no],
-	[AC_COMPILE_IFELSE(
+	[AC_LINK_IFELSE(
           [AC_LANG_PROGRAM([dnl
         #include <stdint.h>
         typedef unsigned uint128_t __attribute__((mode(TI)));