If libdw is not installed in build environment, the output of
'pkg-config --modversion libdw' is empty, causing LIBDW_VERSION_2 to be
empty and the shell test will have the following error:

  /bin/sh: 1: test: -lt: unexpected operator

Before:

  $ pkg-config --modversion libdw
  Package libdw was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libdw.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'libdw' found
  $ make LDFLAGS=-static -j16
    BUILD:   Doing 'make -j20' parallel build
  <SNIP>
  Package libdw was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libdw.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'libdw' found
  /bin/sh: 1: test: -lt: unexpected operator

After:

  1. libdw is not installed:

  $ pkg-config --modversion libdw
  Package libdw was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libdw.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'libdw' found
  $ make LDFLAGS=-static -j16
    BUILD:   Doing 'make -j20' parallel build
  <SNIP>
  Package libdw was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libdw.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'libdw' found
  Makefile.config:473: No libdw DWARF unwind found, Please install elfutils-devel/libdw-dev >= 0.158 and/or set LIBDW_DIR

  2. libdw version is lower than 0.177

  $ pkg-config --modversion libdw
  0.176
  $ make LDFLAGS=-static -j16
    BUILD:   Doing 'make -j20' parallel build
  <SNIP>

  Auto-detecting system features:
  ...                                   dwarf: [ on  ]
  <SNIP>
    INSTALL libsubcmd_headers
    INSTALL libapi_headers
    INSTALL libperf_headers
    INSTALL libsymbol_headers
    INSTALL libbpf_headers
    LINK    perf

  3. libdw version is higher than 0.177

  $ pkg-config --modversion libdw
  0.186
  $ make LDFLAGS=-static -j16
    BUILD:   Doing 'make -j20' parallel build
  <SNIP>

  Auto-detecting system features:
  ...                                   dwarf: [ on  ]
  <SNIP>
    CC      util/bpf-utils.o
    CC      util/pfm.o
    LD      util/perf-util-in.o
    LD      perf-util-in.o
    AR      libperf-util.a
    LINK    perf

Fixes: 536661da ("perf: build: Only link libebl.a for old libdw")
Reviewed-by: Leo Yan <leo.yan@arm.com>
Signed-off-by: Yang Jihong <yangjihong@bytedance.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240919013513.118527-2-yangjihong@bytedance.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>