Allow for running secp256k1 java build/tests out of tree
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

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