dtkComposer requires dtkMathSupport

Created by: jwintz

If dtkComposerMathSupport is not enabled, dtkComposer fails to build with the following message.

[ 25%] Building CXX object src/dtkComposer/CMakeFiles/dtkComposer.dir/dtkComposerEvaluator.cpp.o
In file included from /Users/jwintz/Development/dtk/src/dtkComposer/dtkComposerEvaluator.cpp:23:
In file included from /Users/jwintz/Development/dtk/src/dtkComposer/dtkComposerGraphEdge.h:25:
/Users/jwintz/Development/dtk/src/dtkMathSupport/dtkGraphEdge.h:18:10: fatal error: 'dtkMathSupportExport.h' file not
      found
#include "dtkMathSupportExport.h"
         ^
1 error generated.

Either the build system should handle this, or dtkComposer should not rely on dtkMathSupport, which I personally think is the best option.