Disable second CUSTOM_DATA_INSTALL_PATH definition

Having CUSTOM_DATA_INSTALL_PATH defined twice was causing lots of warnings
while building. Commenting it out means they are stopped but I haven't
determined if that causes other problems during compile.

Example of the warning:
In file included from <built-in>:175:
<command line>:8:9: warning: 'CUSTOM_DATA_INSTALL_PATH' macro redefined
        ^
<command line>:2:9: note: previous definition is here
        ^

Suggestion for how to fix was taken from
https://bugzilla.mozilla.org/show_bug.cgi?id=920108
Set wxWidgets_CONFIG_EXECUTABLE path

For some reason wxWidgets_CONFIG_EXECUTABLE isn't being found by cmake while
building. I'm hard coding it in to place to alow the build to progress.
Set CMAKE_INSTALL_PREFIX to empty

This empty value means paths will work correctly as relative paths when
building locally.
This change cannot be pushed.
Remove MEGAGLEST_VERSION for MG_VERSION_H_CONTENTS

I noticed that mk/macos/CMakeLists.txt has special code for extracting the
megaglest version , but its already done in CMakeLists.txt. This removes the
duplicate detection and changes the users of MEGAGLEST_VERSION to
MG_VERSION_H_CONTENTS.
Change library search path on MacOS

When using mac ports GL/gl.h ends up outside the cmake search paths for
libraries and includes. By adding them to mk/macos/CMakeLists.txt they are now
available and make it possible to build g3d_viewer along with the rest of
megaglest. To make this work properly I had to move the inclusion of
mk/macos/CMakeLists.txt further up the master CMakeLists.txt so merged both IF
APPLE tests in to one.

This has built successfully through xcode and gnumake.