how to make jni.h be found?

Installing the OpenJDK Development Kit (JDK) should fix your problem.

sudo apt-get install openjdk-X-jdk

This should make you able to compile without problems.

It needs both jni.h and jni_md.h files, Try this

gcc -I/usr/lib/jvm/jdk1.7.0_07/include \
  -I/usr/lib/jvm/jdk1.7.0_07/include/linux filename.c

This will include both the broad JNI files and the ones necessary for linux

You have to tell your compiler where is the include directory. Something like this:

gcc -I/usr/lib/jvm/jdk1.7.0_07/include

But it depends on your makefile.

Use the following code:

make -I/usr/lib/jvm/jdk*/include

where jdk* is the directory name of your jdk installation (e.g. jdk1.7.0).

And there wouldn't be a system-wide solution since the directory name would be different with different builds of JDK downloaded and installed. If you desire an automated solution, please include all commands in a single script and run the said script in Terminal.