Fixed some make and linking issues of RocksDBJava

Summary:
Fixed some make and linking issues of RocksDBJava. Specifically:
* Add JAVA_LDFLAGS, which does not include gflags
* rocksdbjava library now uses JAVA_LDFLAGS instead of LDFLAGS
* java/Makefile now includes build_config.mk
* rearrange make rocksdbjava workflow to ensure the library file is correctly
  included in the jar file.

Test Plan:
make rocksdbjava
make jdb_bench
java/jdb_bench.sh

Reviewers: dhruba, swapnilghike, zzbennett, rsumbaly, ankgup87

Reviewed By: ankgup87

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20289

java/Makefile

@@ -3,11 +3,6 @@ NATIVE_JAVA_CLASSES = org.rocksdb.RocksDB org.rocksdb.Options org.rocksdb.WriteB
 NATIVE_INCLUDE = ./include
 ROCKSDB_JAR = rocksdbjni.jar
 
-ROCKSDBJNILIB = ./librocksdbjni.so
-ifeq ($(PLATFORM), OS_MACOSX)
-  ROCKSDBJNILIB = ./librocksdbjni.jnilib
-endif
-
 clean:
 	-find . -name "*.class" -exec rm {} \;
 	-find . -name "hs*.log" -exec rm {} \;
@@ -16,7 +11,6 @@ clean:
 java:
 	javac org/rocksdb/util/*.java org/rocksdb/*.java
 	@cp ../HISTORY.md ./HISTORY-CPP.md
-	jar -cf $(ROCKSDB_JAR) org/rocksdb/*.class org/rocksdb/util/*.class HISTORY*.md $(ROCKSDBJNILIB)
 	@rm -f ./HISTORY-CPP.md
 	javah -d $(NATIVE_INCLUDE) -jni $(NATIVE_JAVA_CLASSES)