RTS configure: Move over `__thread` check

This used by (@bgamari thinks) the `GCThread` abstraction in the RTS.

All of this should boil down to `AC_DEFINE` not `AC_SUBST`, so it
belongs in the RTS configure and should be safe to move without
modification.

Progress towards #17191

configure.ac

@@ -1046,19 +1046,6 @@ AC_CHECK_FUNCS([eventfd])
 
 AC_CHECK_FUNCS([getpid getuid raise])
 
-dnl ** Check for __thread support in the compiler
-AC_MSG_CHECKING(for __thread support)
-AC_COMPILE_IFELSE(
-  [ AC_LANG_SOURCE([[__thread int tester = 0;]]) ],
-  [
-   AC_MSG_RESULT(yes)
-   AC_DEFINE([CC_SUPPORTS_TLS],[1],[Define to 1 if __thread is supported])
-  ],
-  [
-   AC_MSG_RESULT(no)
-   AC_DEFINE([CC_SUPPORTS_TLS],[0],[Define to 1 if __thread is supported])
-  ])
-
 GHC_ADJUSTORS_METHOD([Target])
 AC_SUBST([UseLibffiForAdjustors])
 

rts/configure.ac

@@ -33,6 +33,19 @@ GHC_CONVERT_PLATFORM_PARTS([host], [Host])
 FPTOOLS_SET_PLATFORM_VARS([host], [Host])
 FPTOOLS_SET_HASKELL_PLATFORM_VARS([Host])
 
+dnl ** Check for __thread support in the compiler
+AC_MSG_CHECKING(for __thread support)
+AC_COMPILE_IFELSE(
+  [ AC_LANG_SOURCE([[__thread int tester = 0;]]) ],
+  [
+   AC_MSG_RESULT(yes)
+   AC_DEFINE([CC_SUPPORTS_TLS],[1],[Define to 1 if __thread is supported])
+  ],
+  [
+   AC_MSG_RESULT(no)
+   AC_DEFINE([CC_SUPPORTS_TLS],[0],[Define to 1 if __thread is supported])
+  ])
+
 dnl large address space support (see rts/include/rts/storage/MBlock.h)
 dnl
 dnl Darwin has vm_allocate/vm_protect