perf callchains: Disable unwind libraries when libelf isn't found

I.e. do the same as when NO_LIBELF is explicitely passed in the 'make'
command line, fixing this:

  Auto-detecting system features:
  ...                         dwarf: [ OFF ]
  ...                         glibc: [ on  ]
  ...                          gtk2: [ OFF ]
  ...                      libaudit: [ OFF ]
  ...                        libbfd: [ OFF ]
  ...                        libelf: [ OFF ]
  ...                     libunwind: [ OFF ]
  ...            libdw-dwarf-unwind: [ OFF ]
  ...     DWARF post unwind library: libdw
<SNIP>
    CC       /tmp/build/perf/util/symbol-minimal.o
    CC       /tmp/build/perf/util/unwind-libdw.o
  arch/x86/util/unwind-libdw.c:1:30: fatal error: elfutils/libdwfl.h: No such file or directory
  compilation terminated.
    CC       /tmp/build/perf/tests/keep-tracking.o
  util/unwind-libdw.c:2:28: fatal error: elfutils/libdw.h: No such file or directory
  compilation terminated.

Cc: Adrian Hunter <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Don Zickus <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>

tools/perf/config/Makefile

@@ -278,6 +278,8 @@ else
       NO_LIBELF := 1
       NO_DWARF := 1
       NO_DEMANGLE := 1
+      NO_LIBUNWIND := 1
+      NO_LIBDW_DWARF_UNWIND := 1
     else
       msg := $(error No gnu/libc-version.h found, please install glibc-dev[el]/glibc-static);
     endif