fix Issue 24294 - ImportC: unrecognized command line option -Wno-builtin-macro-redefined with gcc

Author: WalterBright

spec/importc.dd

@@ -150,10 +150,6 @@ $(H2 $(LNAME2 preprocessor, Preprocessor))
     $(LI Otherwise the $(TT cpp) preprocessor will be used.)
     )
 
-    $(P The druntime file $(LINK2 https://github.com/dlang/dmd/blob/master/druntime/src/importc.h, $(TT src/importc.h))
-    will automatically be $(TT #include)d first. $(TT importc.h) provides adjustments to the source code to
-    account for various C compiler extensions not supported by ImportC.)
-
     $(P The $(DDSUBLINK dmd, switch-v, $(TT -v)) switch can be used to observe the command
     that invokes the preprocessor.
     )
@@ -162,6 +158,19 @@ $(H2 $(LNAME2 preprocessor, Preprocessor))
     to the preprocessor.
     )
 
+    $(H4 $(LNAME2 importc-h, $(TT importc.h)))
+
+    $(P The druntime file $(LINK2 https://github.com/dlang/dmd/blob/master/druntime/src/importc.h, $(TT src/importc.h))
+    will automatically be $(TT #include)d first. $(TT importc.h) provides adjustments to the source code to
+    account for various C compiler extensions not supported by ImportC.)
+
+    $(H4 $(LNAME2 no-builtin-macro-redefined, no-builtin-macro-redefined))
+
+    $(P On Posix systems, ImportC will pass the switch $(TT -Wno-builtin-macro-redefined) to the C preprocessor
+    used by $(TT gcc) and $(TT clang).
+    This $(LINK2 https://gcc.gnu.org/legacy-ml/gcc-patches/2008-07/msg02321.html, switch)
+    does not exist in $(TT gcc) preprocessors made before 2008.
+    The workaround is to run the preprocessor manually.)
 
     $(H3 $(LNAME2 manual-cpp, Running the Preprocessor Manually))