perf build: Conditionally add feature check flags for libtrace{event,fs}

This avoids reported warnings when the packages are not installed.

[namhyung]: Removed the dummy assignment and unnecessary ifeq checks.

Fixes: 0f0e1f4 ("perf build: Use pkg-config for feature check for libtrace{event,fs}")
Signed-off-by: Guilherme Amadio <amadio@gentoo.org>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/20240628203432.3273625-1-amadio@gentoo.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Author: amadio

tools/perf/Makefile.config

@@ -182,13 +182,19 @@ endif
 FEATURE_CHECK_CFLAGS-libzstd := $(LIBZSTD_CFLAGS)
 FEATURE_CHECK_LDFLAGS-libzstd := $(LIBZSTD_LDFLAGS)
 
+ifneq ($(NO_LIBTRACEEVENT),1)
+  ifeq ($(call get-executable,$(PKG_CONFIG)),)
+    $(error Error: $(PKG_CONFIG) needed by libtraceevent is missing on this system, please install it)
+  endif
+endif
+
 # for linking with debug library, run like:
 # make DEBUG=1 PKG_CONFIG_PATH=/opt/libtraceevent/(lib|lib64)/pkgconfig
-FEATURE_CHECK_CFLAGS-libtraceevent := $(shell $(PKG_CONFIG) --cflags libtraceevent)
-FEATURE_CHECK_LDFLAGS-libtraceevent := $(shell $(PKG_CONFIG) --libs libtraceevent)
+FEATURE_CHECK_CFLAGS-libtraceevent := $(shell $(PKG_CONFIG) --cflags libtraceevent 2>/dev/null)
+FEATURE_CHECK_LDFLAGS-libtraceevent := $(shell $(PKG_CONFIG) --libs libtraceevent 2>/dev/null)
 
-FEATURE_CHECK_CFLAGS-libtracefs := $(shell $(PKG_CONFIG) --cflags libtracefs)
-FEATURE_CHECK_LDFLAGS-libtracefs := $(shell $(PKG_CONFIG) --libs libtracefs)
+FEATURE_CHECK_CFLAGS-libtracefs := $(shell $(PKG_CONFIG) --cflags libtracefs 2>/dev/null)
+FEATURE_CHECK_LDFLAGS-libtracefs := $(shell $(PKG_CONFIG) --libs libtracefs 2>/dev/null)
 
 FEATURE_CHECK_CFLAGS-bpf = -I. -I$(srctree)/tools/include -I$(srctree)/tools/arch/$(SRCARCH)/include/uapi -I$(srctree)/tools/include/uapi
 # include ARCH specific config
@@ -208,12 +214,6 @@ ifeq ($(call get-executable,$(BISON)),)
   $(error Error: $(BISON) is missing on this system, please install it)
 endif
 
-ifneq ($(NO_LIBTRACEEVENT),1)
-  ifeq ($(call get-executable,$(PKG_CONFIG)),)
-  dummy := $(error Error: $(PKG_CONFIG) needed by libtraceevent is missing on this system, please install it)
-  endif
-endif
-
 ifneq ($(OUTPUT),)
   ifeq ($(shell expr $(shell $(BISON) --version | grep bison | sed -e 's/.\+ \([0-9]\+\).\([0-9]\+\).\([0-9]\+\)/\1\2\3/g') \>\= 371), 1)
     BISON_FILE_PREFIX_MAP := --file-prefix-map=$(OUTPUT)=