patch 8.2.3038: Amiga built-in version string doesn't include build date

Problem:    Amiga built-in version string doesn't include build date.
Solution:   Add the build date if available. (Ola Söder, closes #8437)
diff --git a/src/os_amiga.c b/src/os_amiga.c
index 91c13e7..c36206c 100644
--- a/src/os_amiga.c
+++ b/src/os_amiga.c
@@ -116,6 +116,9 @@
 # ifdef PATCHLEVEL
     "." PATCHLEVEL
 # endif
+# ifdef BUILDDATE
+    " (" BUILDDATE ")"
+# endif
     ;
 #endif