Re-apply unmerged secp-zkp patch

commit 14be601
Author: Jon Griffiths <jon_p_griffiths@yahoo.com>
Date:   Sun Jul 9 18:13:00 2017 +1200

    secp: Don't fail JNI dependency checks if we can build a JNI program

    Not all setups will result in JNI_INCLUDE_DIRS having a value, in
    particular android builds from installed toolchain directories have
    the JNI headers installed in the standard include directories.

    Instead of failing if the configure macro didn't to find a directory
    based on the installed java runtime, only fail if we cannot build a
    JNI-using program. This works better for cross compiling and setups
    where the the user wants to compile with one Java and build against
    another.

    This will be submitted upstream in due course.

Author: Lawrence Nahum

src/secp256k1/configure.ac

@@ -483,6 +483,9 @@ if test x"$use_jni" != x"no"; then
   if test "x$JNI_INCLUDE_DIRS" = "x"; then
     have_jni_dependencies=no
   fi
+  if test "x$have_jni_dependencies" = "xno"; then
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <jni.h>]])], [have_jni_dependencies=yes])
+  fi
   if test "x$have_jni_dependencies" = "xno"; then
     if test x"$use_jni" = x"yes"; then
       AC_MSG_ERROR([jni support explicitly requested but headers/dependencies were not found. Enable ECDH and try again.])