Mongoose: Define MONGOOSE_BUILDING when building objects for .mex

`.mex` files are basically shared libraries that are loaded by Matlab.
Define `MONGOOSE_BUILDING` while building the object files for the
`.mex` file.

Mongoose/MATLAB/mongoose_make.m

@@ -38,6 +38,9 @@ function mongoose_make (run_test)
 % Append optimization
 flags = [flags ' -O -silent COPTIMFLAGS="-O3 -fwrapv"'];
 
+% Append while building objects for shared library
+flags = [flags ' -DMONGOOSE_BUILDING'];
+
 cpp_flags = '' ;
 lib = '';
 if (isunix)