Support standard Oracle JDK installation layout on Mac OS X.

Hello,

I tried to use AX_JNI_INCLUDE_DIR in Mac OS X and failed. Reading the implementation, I found that it basically assumes that JDK 6 is being used, as packaged by Apple. Apple's packaging differs from others, in that headers are NOT placed in $JAVA_HOME/include and so it fails to locate them on my OS X Yosemite machine with Oracle JDK 8 installed (also tried Oracle JDK 7 and got the same failure).

Since JDK 7 this is no longer the case on OS X, as Apple no longer provides its own packages (most users install Oracle's JDK). Therefore AX_JNI_INCLUDE_DIR fails to locate the headers (which are now in the "standard" $JAVA_HOME/include location and the special-handling code for "darwin" fails to find them).

This patch adjust the "darwin" special-case handling as follows: it checks to see if the detected JDK is located in /System/Library/Frameworks/JavaVM.framework (this is where Apple installs its package) and only uses the special-handling for that case. Otherwise it reverts to "default handling" as for all other platforms. This is the only way I could think of to "reliably" determine if we are talking about packaging (Apple reserves /System/Library for its own packages).

IMPORTANT: I am not an M4 expert, have never contributed to autoconf-archive and the patch is a best-effort attempt to provide implementation for something that I feel was "too trivial to submit a feature request for without feeling embarrassed". Please feel free to implement from scratch and consider this an "informed request".