perf tools: Add LIBDW_DIR Makefile variable to for alternate libdw

Adding LIBDW_DIR Makefile variable to be able to specify
alternate libdw library location.

To use it run make like:
  $ make LIBDW_DIR=/opt/libdw/
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-n2uv8c9ti6b26fioaw2rq5yv@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile

@@ -501,7 +501,14 @@ ifneq ($(call try-cc,$(SOURCE_LIBELF),$(FLAGS_LIBELF),libelf),y)
 		msg := $(error No gnu/libc-version.h found, please install glibc-dev[el]/glibc-static);
 	endif
 else
-	FLAGS_DWARF=$(ALL_CFLAGS) -ldw -lelf $(ALL_LDFLAGS) $(EXTLIBS)
+	# for linking with debug library, run like:
+	# make DEBUG=1 LIBDW_DIR=/opt/libdw/
+	ifdef LIBDW_DIR
+		LIBDW_CFLAGS  := -I$(LIBDW_DIR)/include
+		LIBDW_LDFLAGS := -L$(LIBDW_DIR)/lib
+	endif
+
+	FLAGS_DWARF=$(ALL_CFLAGS) $(LIBDW_CFLAGS) -ldw -lelf $(LIBDW_LDFLAGS) $(ALL_LDFLAGS) $(EXTLIBS)
 	ifneq ($(call try-cc,$(SOURCE_DWARF),$(FLAGS_DWARF),libdw),y)
 		msg := $(warning No libdw.h found or old libdw.h found or elfutils is older than 0.138, disables dwarf support. Please install new elfutils-devel/libdw-dev);
 		NO_DWARF := 1
@@ -556,7 +563,8 @@ ifndef NO_DWARF
 ifeq ($(origin PERF_HAVE_DWARF_REGS), undefined)
 	msg := $(warning DWARF register mappings have not been defined for architecture $(ARCH), DWARF support disabled);
 else
-	BASIC_CFLAGS += -DDWARF_SUPPORT
+	BASIC_CFLAGS := -DDWARF_SUPPORT $(LIBDW_CFLAGS) $(BASIC_CFLAGS)
+	BASIC_LDFLAGS := $(LIBDW_LDFLAGS) $(BASIC_LDFLAGS)
 	EXTLIBS += -lelf -ldw
 	LIB_OBJS += $(OUTPUT)util/probe-finder.o
 	LIB_OBJS += $(OUTPUT)util/dwarf-aux.o