Use std math functions in Max plugin.

VS12 provides math functions which were conflicting with ones defined
in texutil.h.  This removes the reliance on texutil.h and uses the
standard library's math in its place.
Add different SetText values to MaxCompat header.

Restores compilation of Max 7 plugin, as its SetText expects
a non-const pointer.

Additionally, NotifyInfo should already be declared in notify.h.
Fix scope on preprocessor variables for Max plugin.

plAudioCore needs to know whether we intend to build the plugin, so
the change introduced in 978edc1 was causing compilation to fail when
building the plugin.

This moves detection and definition back into the root CMakeList,
but preserves the behavior of only looking for the SDK if needed.
Merge pull request #406 from Deledrius/Max7_VS12

Fix compilation of Max 7 plugin in VisualStudio 12