Prevent -Werror,-Wunknown-warning-option when building compilerplugins

...with --enable-werror (seen it fail with a local build against a locally
built Clang 5.0.2).

(bin/gen-boost-headers faces a similar dilemma with Clang needing to silence
-Wunknown-warning-option and GCC failing upon the silencing incantation.  There,
we were able to hack around that with a preceding

#pragma GCC diagnostic ignored "-Wpragmas"

Here, the easiest approach appears to be a new COMPILER_PLUGINS_COM_IS_CLANG
analoguous to the existing COM_IS_CLANG.)

Change-Id: I9036261fdd238c8a020a1d88b4e75fd444f9e030
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/88725
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/compilerplugins/Makefile-clang.mk b/compilerplugins/Makefile-clang.mk
index 543ec27aad44..266919cf02b8 100644
--- a/compilerplugins/Makefile-clang.mk
+++ b/compilerplugins/Makefile-clang.mk
@@ -101,6 +101,14 @@ CLANGWERROR :=
 #TODO: /WX
 else
 CLANGWERROR := -Werror
+# When COMPILER_PLUGINS_CXXFLAGS (obtained via `llvm-config --cxxflags`) contains options like
+# -Wno-maybe-uninitialized that are targeting GCC (when LLVM was actually built with GCC), and
+# COMPILER_PLUGINS_CXX (defaulting to CXX) deonotes a Clang that does not understand those options,
+# it fails with -Werror,-Wunknown-warning-option, so we need -Wno-unknown-warning-option (but which
+# GCC does not understand) at least with -Werror:
+ifeq ($(COMPILER_PLUGINS_COM_IS_CLANG),TRUE)
+CLANGWERROR += -Wno-unknown-warning-option
+endif
 endif
 endif