[PATCH bpf v2] tools: fix unavoidable GCC call in Clang builds

[Adrian Ratiu]

In ChromeOS and Gentoo we catch any unwanted mixed Clang/LLVM
and GCC/binutils usage via toolchain wrappers which fail builds.
This has revealed that GCC is called unconditionally in Clang
configured builds to populate GCC_TOOLCHAIN_DIR.

Allow the user to override CLANG_CROSS_FLAGS to avoid the GCC
call - in our case we set the var directly in the ebuild recipe.

In theory Clang could be able to autodetect these settings so
this logic could be removed entirely, but in practice as the
commit cebdb7374577 ("tools: Help cross-building with clang")
mentions, this does not always work, so giving distributions
more control to specify their flags & sysroot is beneficial.

Suggested-by: Manoj Gupta <manojgupta@xxxxxxxxxxxx>
Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Acked-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx>
---
Changes in v2:
  * Replaced variable override GCC_TOOLCHAIN_DIR -> CLANG_CROSS_FLAGS
---
 tools/scripts/Makefile.include | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
index 79d102304470..b9b1deacc4eb 100644
--- a/tools/scripts/Makefile.include
+++ b/tools/scripts/Makefile.include
@@ -89,6 +89,9 @@ ifeq ($(CC_NO_CLANG), 1)
 EXTRA_WARNINGS += -Wstrict-aliasing=3
 
 else ifneq ($(CROSS_COMPILE),)
+# allow userspace to override CLANG_CROSS_FLAGS to specify their own
+# sysroots and flags or to avoid the GCC call in pure Clang builds
+ifeq ($(CLANG_CROSS_FLAGS),)
 CLANG_CROSS_FLAGS := --target=$(notdir $(CROSS_COMPILE:%-=%))
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)gcc 2>/dev/null))
 ifneq ($(GCC_TOOLCHAIN_DIR),)
@@ -96,6 +99,7 @@ CLANG_CROSS_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
 CLANG_CROSS_FLAGS += --sysroot=$(shell $(CROSS_COMPILE)gcc -print-sysroot)
 CLANG_CROSS_FLAGS += --gcc-toolchain=$(realpath $(GCC_TOOLCHAIN_DIR)/..)
 endif # GCC_TOOLCHAIN_DIR
+endif # CLANG_CROSS_FLAGS
 CFLAGS += $(CLANG_CROSS_FLAGS)
 AFLAGS += $(CLANG_CROSS_FLAGS)
 endif # CROSS_COMPILE
-- 
2.35.1