Revert "Remove broken rebuild of compilerplugins when CLANG_FULL_VERSION..."

This reverts commit efe2889549.
It is that commit that is broken. The only thing that does not work is
that not all compilerplugin sources depend on config_clang.h, the rest is fine.
So instead of reverting something that in principle works (and even complaining
in the commit message about the original problem), just fix it
(will do in next commit).

Change-Id: Ic7766a97220d5b7ef1cd195320899564140fdf1c
Reviewed-on: https://gerrit.libreoffice.org/72975
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

compilerplugins/clang/plugin.hxx

@@ -12,6 +12,8 @@
 #ifndef PLUGIN_H
 #define PLUGIN_H
 
+#include <config_clang.h>
+
 #include <clang/AST/ASTContext.h>
 #include <clang/AST/RecursiveASTVisitor.h>
 #include <clang/Basic/FileManager.h>