add -lm to DUMB_LIBRARIES

Without -lm, cmake finds the include paths and libraries, can compile
the test program, but cannot link it. Then, cmake declares that DUMB is
not installed despite properly installed DUMB headers and libs.

Tested on Arch with the dumb 1.0-1 package. Unsure if this is necessary
on other systems, and unsure whether it flat-out breaks the build on
other systems.

cmake/FindDUMB.cmake

@@ -21,7 +21,7 @@ find_package_handle_standard_args(DUMB DEFAULT_MSG
     DUMB_INCLUDE_DIR DUMB_LIBRARY)
 
 if(DUMB_FOUND)
-  set(DUMB_LIBRARIES ${DUMB_LIBRARY})
+  set(DUMB_LIBRARIES ${DUMB_LIBRARY};m)
 else(DUMB_FOUND)
   set(DUMB_LIBRARIES)
 endif(DUMB_FOUND)