Merge bitcoin#460: [build] Update ax_jni_include_dir.m4 macro

e7daa9b [build] Tweak JNI macro to warn instead of error for JNI not found. (Karl-Johan Alm)
5b22977 [build] Update ax_jni_include_dir.m4 macro to deal with recent versions of macOS (Karl-Johan Alm)

Pull request description:

  Prior to this patch, this macro fails to find a working directory for the JNI headers, and results in compile failure when doing
  ```
  ./configure --enable-experimental --enable-module-ecdh --enable-jni
  ```
  on more recent macOS versions. The relevant commit upstream is [here](http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commit;h=ab23d25b1f1ae544fffdaa0a94a794798695c672) from the [GNU.org page for the macro](https://www.gnu.org/software/autoconf-archive/ax_jni_include_dir.html). The original (identical to the version in this commit) is [here](http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_jni_include_dir.m4).

  The compile failure:
  ```Bash
  $ make
  gcc -I. -g -O2 -Wall -Wextra -Wno-unused-function -c src/gen_context.c -o gen_context.o
  gcc gen_context.o -o gen_context
  ./gen_context
    CC       src/libsecp256k1_la-secp256k1.lo
    CC       src/java/libsecp256k1_jni_la-org_bitcoin_NativeSecp256k1.lo
  In file included from src/java/org_bitcoin_NativeSecp256k1.c:4:
  In file included from src/java/org_bitcoin_NativeSecp256k1.h:2:
  /Library/Java/JavaVirtualMachines/jdk1.7.0_71.jdk/Contents/Headers/jni.h:45:10: fatal error:
        'jni_md.h' file not found
  #include "jni_md.h"
           ^
  1 error generated.
  make: *** [src/java/libsecp256k1_jni_la-org_bitcoin_NativeSecp256k1.lo] Error 1
  ```

Tree-SHA512: 9a8305b3ed67eba0de728f91cf435857a676ba10507ab8481a3c03b50e1ce0469a3d79e751d0a697018789f21e2aa48b7eccca4d225520a3863fcf23f1fd487a

Author: gmaxwell

build-aux/m4/ax_jni_include_dir.m4

@@ -1,5 +1,5 @@
 # ===========================================================================
-#    http://www.gnu.org/software/autoconf-archive/ax_jni_include_dir.html
+#    https://www.gnu.org/software/autoconf-archive/ax_jni_include_dir.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -44,7 +44,7 @@
 #   and this notice are preserved. This file is offered as-is, without any
 #   warranty.
 
-#serial 10
+#serial 14
 
 AU_ALIAS([AC_JNI_INCLUDE_DIR], [AX_JNI_INCLUDE_DIR])
 AC_DEFUN([AX_JNI_INCLUDE_DIR],[
@@ -66,40 +66,45 @@ else
 fi
 
 case "$host_os" in
-        darwin*)        _JTOPDIR=`echo "$_JTOPDIR" | sed -e 's:/[[^/]]*$::'`
-                        _JINC="$_JTOPDIR/Headers";;
-        *)              _JINC="$_JTOPDIR/include";;
+        darwin*)        # Apple Java headers are inside the Xcode bundle.
+            macos_version=$(sw_vers -productVersion | sed -n -e 's/^@<:@0-9@:>@*.\(@<:@0-9@:>@*\).@<:@0-9@:>@*/\1/p')
+            if @<:@ "$macos_version" -gt "7" @:>@; then
+                _JTOPDIR="$(xcrun --show-sdk-path)/System/Library/Frameworks/JavaVM.framework"
+                _JINC="$_JTOPDIR/Headers"
+            else
+                _JTOPDIR="/System/Library/Frameworks/JavaVM.framework"
+                _JINC="$_JTOPDIR/Headers"
+            fi
+            ;;
+        *) _JINC="$_JTOPDIR/include";;
 esac
 _AS_ECHO_LOG([_JTOPDIR=$_JTOPDIR])
 _AS_ECHO_LOG([_JINC=$_JINC])
 
 # On Mac OS X 10.6.4, jni.h is a symlink:
 # /System/Library/Frameworks/JavaVM.framework/Versions/Current/Headers/jni.h
 # -> ../../CurrentJDK/Headers/jni.h.
-
 AC_CACHE_CHECK(jni headers, ac_cv_jni_header_path,
 [
-if test -f "$_JINC/jni.h"; then
-  ac_cv_jni_header_path="$_JINC"
-  JNI_INCLUDE_DIRS="$JNI_INCLUDE_DIRS $ac_cv_jni_header_path"
-else
-  _JTOPDIR=`echo "$_JTOPDIR" | sed -e 's:/[[^/]]*$::'`
-  if test -f "$_JTOPDIR/include/jni.h"; then
-    ac_cv_jni_header_path="$_JTOPDIR/include"
+  if test -f "$_JINC/jni.h"; then
+    ac_cv_jni_header_path="$_JINC"
     JNI_INCLUDE_DIRS="$JNI_INCLUDE_DIRS $ac_cv_jni_header_path"
   else
-    ac_cv_jni_header_path=none
+    _JTOPDIR=`echo "$_JTOPDIR" | sed -e 's:/[[^/]]*$::'`
+    if test -f "$_JTOPDIR/include/jni.h"; then
+      ac_cv_jni_header_path="$_JTOPDIR/include"
+      JNI_INCLUDE_DIRS="$JNI_INCLUDE_DIRS $ac_cv_jni_header_path"
+    else
+      ac_cv_jni_header_path=none
+    fi
   fi
-fi
 ])
 
-
-
 # get the likely subdirectories for system specific java includes
 case "$host_os" in
 bsdi*)          _JNI_INC_SUBDIRS="bsdos";;
-darwin*)        _JNI_INC_SUBDIRS="darwin";;
 freebsd*)       _JNI_INC_SUBDIRS="freebsd";;
+darwin*)        _JNI_INC_SUBDIRS="darwin";;
 linux*)         _JNI_INC_SUBDIRS="linux genunix";;
 osf*)           _JNI_INC_SUBDIRS="alpha";;
 solaris*)       _JNI_INC_SUBDIRS="solaris";;
@@ -112,9 +117,9 @@ if test "x$ac_cv_jni_header_path" != "xnone"; then
   # add any subdirectories that are present
   for JINCSUBDIR in $_JNI_INC_SUBDIRS
   do
-      if test -d "$_JTOPDIR/include/$JINCSUBDIR"; then
-           JNI_INCLUDE_DIRS="$JNI_INCLUDE_DIRS $_JTOPDIR/include/$JINCSUBDIR"
-      fi
+    if test -d "$_JTOPDIR/include/$JINCSUBDIR"; then
+         JNI_INCLUDE_DIRS="$JNI_INCLUDE_DIRS $_JTOPDIR/include/$JINCSUBDIR"
+    fi
   done
 fi
 ])