Makefile: Do not use GCC specific options with LLVM/clang

Building with LLVM/clang (`COMPILER_LLVM_CLANG=y`), Debian clang version
11.0.1-2 fails due to unknown warning options.

    error: unknown warning option '-Wlogical-op'; did you mean '-Wlong-long'? [-Werror,-Wunknown-warning-option]
    error: unknown warning option '-Wduplicated-cond' [-Werror,-Wunknown-warning-option]

As these are GCC specific, only add them, when building with GCC (and
not scan-build).

Fixes: 04e0712f46 ("Treewide: Add some gcc's warning options")
Change-Id: I6190c1f3df97fb0be51f8dab7e1f5f2a033f5d86
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50771
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/Makefile.inc b/Makefile.inc
index a57fac0..f46de46 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -426,7 +426,7 @@
 CFLAGS_common += -nostdlib -Wall -Wundef -Wstrict-prototypes -Wmissing-prototypes
 CFLAGS_common += -Wwrite-strings -Wredundant-decls -Wno-trigraphs -Wimplicit-fallthrough
 CFLAGS_common += -Wshadow -Wdate-time -Wtype-limits -Wvla
-CFLAGS_common += -Wlogical-op -Wduplicated-cond -Wdangling-else
+CFLAGS_common += -Wdangling-else
 CFLAGS_common += -fno-common -ffreestanding -fno-builtin -fomit-frame-pointer
 CFLAGS_common += -fstrict-aliasing -ffunction-sections -fdata-sections -fno-pie
 ifeq ($(CONFIG_COMPILER_GCC),y)
@@ -435,6 +435,7 @@
 CFLAGS_common += -Wno-packed-not-aligned
 CFLAGS_common += -fconserve-stack
 CFLAGS_common += -Wnull-dereference -Wreturn-type
+CFLAGS_common += -Wlogical-op -Wduplicated-cond
 # cf. commit f69a99db (coreboot: x86: enable gc-sections)
 CFLAGS_common += -Wno-unused-but-set-variable
 endif