cmake: Do add dependencies

Unfortunately ninja does not want to be tricked.

I tried `touch`ing a file and writing the date to a file,
and even removing that file before cargo runs, it doesn't work.

So instead we'll do the imperfect solution of enumerating sources.

And yes, we use a GLOB because listing source files is terrible.
Any build system that wants you not to glob is a build system made for
build system people who like touching build systems, not me.

CMakeLists.txt

@@ -93,16 +93,10 @@ add_definitions(-DCMAKE_SOURCE_DIR="${REAL_CMAKE_SOURCE_DIR}")
 # Enable thread-safe errno on Solaris (#5611)
 add_definitions(-D_REENTRANT)
 
-# Cmake doesn't know when cargo should be rerun.
-# We could enumerate when (source files changed, Cargo.toml/lock, cargo update, ...),
-# but that's a pain and cargo takes <100ms when there's nothing to do,
-# so we can just always rerun it.
-# So this adds a dummy command that is successful but creates no output,
-# and then depend on it.
-add_custom_command(
-  OUTPUT always_rebuild
-  COMMAND echo
-)
+# cargo needs to be rerun when the sources change.
+# This is imperfect, but the ninja generator really wants to
+# not run cargo, so we need to tell it *something*
+FILE(GLOB sources src/*)
 
 add_custom_command(
   OUTPUT ${rust_target_dir}/${rust_profile}/libfish.rlib
@@ -117,7 +111,8 @@ add_custom_command(
       ${FEATURES_ARG}
   WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
   USES_TERMINAL
-  DEPENDS always_rebuild
+  DEPENDS ${sources}
+  DEPENDS Cargo.toml Cargo.lock build.rs
   # Don't use VERBATIM here, to allow the generator expressions above to expand to nothing rather than an empty string
   )
 add_custom_target(libfish DEPENDS ${rust_target_dir}/${rust_profile}/libfish.rlib)