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In some distros slang.h may be in a /usr/include 'slang' subdir, so use
the if slang is not explicitely disabled (by using NO_SLANG=1) and its
feature test for the common case (having /usr/include/slang.h) failed,
use the results for the test that checks if it is in slang/slang.h.

Change the only file in perf that includes slang.h to use
HAVE_SLANG_INCLUDE_SUBDIR and forget about this for good.

On a rhel6 system now we have:

  $ /tmp/build/perf/perf -vv | grep slang
                libslang: [ on  ]  # HAVE_SLANG_SUPPORT
  $ ldd /tmp/build/perf/perf | grep libslang
  	libslang.so.2 => /usr/lib64/libslang.so.2 (0x00007fa2d5a8d000)
  $ grep slang /tmp/build/perf/FEATURE-DUMP
  feature-libslang=0
  feature-libslang-include-subdir=1
  $ cat /etc/redhat-release
  CentOS release 6.10 (Final)
  $

While on fedora:29:

  $ /tmp/build/perf/perf -vv | grep slang
                libslang: [ on  ]  # HAVE_SLANG_SUPPORT
  $ ldd /tmp/build/per...