Improve build parallelism by compiling mrregister and mrtransform first

mrregister and mrtransform can take a long time to build. If their compilation starts towards the end of the build, Ninja may be compiling a single source file for quite sometime. This may result in suboptimal compilation times.
This hack tries to avoid this by starting their compilation as soon as possible (atm there's no alternative nice way of doing this in Ninja or CMake).

See ninja-build/ninja#232

cmd/CMakeLists.txt

@@ -26,8 +26,14 @@ foreach(CMD ${HEADLESS_CMD_SRCS})
     add_cmd(${CMD} FALSE)
 endforeach(CMD)
 
+# mrregister and mrtransform are the commands that take the longest to compile
+# so we try to start their compilation as soon as possible by adding them as dependencies
+# of the first command in alphabetical order. This is hacky, but it provides a significant
+# speedup in the compilation time when using Ninja.
+add_dependencies(5tt2gmwmi mrregister mrtransform)
+
 if(MRTRIX_BUILD_GUI)
     foreach(CMD ${GUI_CMD_SRCS})
         add_cmd(${CMD} TRUE)
     endforeach(CMD)
-endif()
+endif()