Allow for running secp256k1 java build/tests out of tree

Summary: As per title

Test Plan:

mkdir -p src/secp256k1/build
cd src/secp256k1
mkdir -p src/java/guava
wget https://search.maven.org/remotecontent?filepath=com/google/guava/guava/18.0/guava-18.0.jar -O src/java/guava/guava-18.0.jar
./autogen.sh
cd build
../configure --enable-jni --enable-experimental --enable-module-ecdh
make check-java

(make check-java may pass or fail depending of D2685)
Use make clean to remove the .class files generated by javac.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2717