Definition of JAVA_HOME32 or JAVA_HOME64 is not working on Linux

[fjtc]

Definition of JAVA_HOME32 or JAVA_HOME64 is not working properly on Linux. This makes the usage of those variables or even the direct definition of JAVA_HOME incompatible with the Linux version.

[fjtc]

I posted a commit that fix the issues for Linux but it is still not tested on Windows.

This patch also makes the following changes in the priorities used to determine the final value of JAVA_HOME:

1. Current value of cmake's variable JAVA_HOME if it is defined;
2. Current value of cmake's variables JAVA_HOME32 or JAVA_HOME64 depending on the target platform;
3. Current value of the environment variable JAVA_HOME;

If JAVA_HOME cannot be determined, or points to an invalid JDK, the module will set JNI_FOUND to 0 instead of 1.

This will increase the methods that can be used to determine where the JDK actually is.