fix macos assemble_gui dependency

The `sign` target on macos was not working properly -- the signing
script would run before the build is finished.  This was caused by
cmake/macos.cmake having an `if(BUILD_GUI)`, but BUILD_GUI isn't defined
as an option until cmake/gui.cmake, which hadn't been included yet where
macos.cmake was included.

This extracts just the `option(BUIL_GUI)` from gui.cmake into a separate
gui-option.cmake file that we can load earlier to fix it.

While here I also noticed the GUI_EXE setting was defined as an option,
but isn't actually a boolean value, as an option, but isn't actually a
boolean value, so fixed it by making it a `set(... CACHE FILEPATH ...)`.

CMakeLists.txt

@@ -114,6 +114,8 @@ if (STATIC_LINK)
   message(STATUS "setting static library suffix search")
 endif()
 
+include(cmake/gui-option.cmake)
+
 include(cmake/solaris.cmake)
 include(cmake/win32.cmake)
 include(cmake/macos.cmake)

cmake/gui-option.cmake

@@ -0,0 +1,6 @@
+set(default_build_gui OFF)
+if(APPLE OR WIN32)
+  set(default_build_gui ON)
+endif()
+
+option(BUILD_GUI "build electron gui from 'gui' submodule source" ${default_build_gui})

cmake/gui.cmake

@@ -1,22 +1,15 @@
-set(default_build_gui OFF)
+
 set(default_gui_target pack)
 if(APPLE)
-  set(default_build_gui ON)
   set(default_gui_target macos:raw)
 elseif(WIN32)
-  set(default_build_gui ON)
   set(default_gui_target win32)
+  set(GUI_EXE "" CACHE FILEPATH "path to an externally built lokinet gui.exe")
 endif()
 
-if(WIN32)
-  option(GUI_EXE "path to an externally built lokinet gui.exe" OFF)
-endif()
-
-option(BUILD_GUI "build electron gui from 'gui' submodule source" ${default_build_gui})
 set(GUI_YARN_TARGET "${default_gui_target}" CACHE STRING "yarn target for building the GUI")
 set(GUI_YARN_EXTRA_OPTS "" CACHE STRING "extra options to pass into the yarn build command")
 
-
 if (BUILD_GUI)
   message(STATUS "Building lokinet-gui")
   # allow manually specifying yarn with -DYARN=