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 (due to part of the compilation not being multi-threaded).
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

@@ -37,8 +37,16 @@ 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 a generated dummy file (which Ninja prioritises during compilation).
+# This is hacky, but it increases build parallelism.
+file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp "int main(){}")
+add_executable(0_dummy ${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp)
+add_dependencies(0_dummy mrregister mrtransform)
+
 if(MRTRIX_BUILD_GUI)
     foreach(CMD ${GUI_CMD_SRCS})
         add_cmd(${CMD} TRUE)
     endforeach(CMD)
-endif()
+endif()