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
cd329dbc3e