Native jni build (#5)

* Each native library is in its own jar. Got rid of cross compilation. The project is `secp256k1` and not `secp256k1-kmp`.

* Updated CI to use all 3 OS VMs

Co-authored-by: Salomon BRYS <salomon@kodein.net>

jni/jvm/build.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+set -e
+
+[[ -z "$TARGET" ]] && echo "Please set the TARGET variable" && exit 1
+
+if [ "$(id -u)" == "0" ]; then
+  [[ -z "$TO_UID" ]] && echo "Please set the TO_UID variable" && exit 1
+fi
+
+cd "$(dirname "$0")"
+
+CC=gcc
+JNI_HEADERS=$TARGET
+
+if [ "$TARGET" == "linux" ]; then
+  OUTFILE=libsecp256k1-jni.so
+  ADD_LIB=-lgmp
+  CC_OPTS="-fPIC"
+elif [ "$TARGET" == "darwin" ]; then
+  OUTFILE=libsecp256k1-jni.dylib
+  ADD_LIB=-lgmp
+elif [ "$TARGET" == "mingw" ]; then
+  OUTFILE=secp256k1-jni.dll
+  CC=x86_64-w64-mingw32-gcc
+fi
+
+mkdir -p build/jni/$TARGET
+
+$CC -shared $CC_OPTS -o build/$TARGET/$OUTFILE ../c/src/fr_acinq_secp256k1_Secp256k1CFunctions.c -I../c/headers/ -I../c/headers/java -I../c/headers/$JNI_HEADERS/ -I../../native/secp256k1/ -lsecp256k1 -L../../native/build/$TARGET/ $ADD_LIB
+
+[[ ! -z "$TO_UID" ]] && chown -R $TO_UID:$TO_UID .
+
+echo "Build done for $TARGET"