[PATCH 2/4] perf tools: Add EXCLUDE_EXTLIBS and EXTRA_PERFLIBS to makefile

[David Carrillo-Cisneros <davidcc@google.com>]

The goal is to allow users to override linking of libraries that
were automatically added to PERFLIBS.

EXCLUDE_EXTLIBS contains linker flags to be removed from LIBS
while EXTRA_PERFLIBS contains linker flags to be added.

My use case is to force certain library to be build statically,
e.g. for libelf:

  EXCLUDE_EXTLIBS=-lelf EXTRA_PERFLIBS=path/libelf.a

Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
---
 tools/perf/Makefile.perf | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index 5008f51a08a2..100a6c1670c8 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -33,6 +33,11 @@ include ../scripts/utilities.mak
 #
 # Define EXTRA_CFLAGS=-m64 or EXTRA_CFLAGS=-m32 as appropriate for cross-builds.
 #
+# Define EXCLUDE_EXTLIBS=-lmylib to exclude libmylib from the auto-generated
+# EXTLIBS.
+#
+# Define EXTRA_PERFLIBS to pass extra libraries to PERFLIBS.
+#
 # Define NO_DWARF if you do not want debug-info analysis feature at all.
 #
 # Define WERROR=0 to disable treating any warnings as errors.
@@ -352,7 +357,8 @@ ifdef ASCIIDOC8
   export ASCIIDOC8
 endif
 
-LIBS = -Wl,--whole-archive $(PERFLIBS) -Wl,--no-whole-archive -Wl,--start-group $(EXTLIBS) -Wl,--end-group
+EXTLIBS := $(call filter-out,$(EXCLUDE_EXTLIBS),$(EXTLIBS))
+LIBS = -Wl,--whole-archive $(PERFLIBS) $(EXTRA_PERFLIBS) -Wl,--no-whole-archive -Wl,--start-group $(EXTLIBS) -Wl,--end-group
 
 ifeq ($(USE_CLANG), 1)
   CLANGLIBS_LIST = AST Basic CodeGen Driver Frontend Lex Tooling Edit Sema Analysis Parse Serialization
-- 
2.13.2.932.g7449e964c-goog