compile pragma: cache the result sooner (#19554)

extccomp.addExternalFileToCompile() relies on hashes to decide whether
an external C file needs recompilation or not.

Due to short-circuit evaluation of boolean expressions, the procedure
that generates a corresponding hash file is not called the first time an
external file is compiled, so an avoidable recompilation is triggered
the next build.

This patch fixes that by moving the proc call with a desired side
effect from its boolean expression, so it's executed unconditionally.

(cherry picked from commit 0c915b5)

compiler/extccomp.nim

@@ -649,8 +649,10 @@ proc externalFileChanged(conf: ConfigRef; cfile: Cfile): bool =
       close(f)
 
 proc addExternalFileToCompile*(conf: ConfigRef; c: var Cfile) =
+  # we want to generate the hash file unconditionally
+  let extFileChanged = externalFileChanged(conf, c)
   if optForceFullMake notin conf.globalOptions and fileExists(c.obj) and
-      not externalFileChanged(conf, c):
+      not extFileChanged:
     c.flags.incl CfileFlag.Cached
   else:
     # make sure Nim keeps recompiling the external file on reruns