[PATCH] elfutils/configure.ac: use $READELF, not readelf

[Sergei Trofimovich <slyfox@gentoo.org>]

Allow user to specify own readelf. Use detected readelf,
not 'readelf'.

Noticed when was building elfutils on tuple-prefixed toolchain:

```
checking whether the compiler generates build-ids...
  ./configure: line 5197: readelf: command not found
no
```

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
---
 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 74cc749d..8d3bcb7b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -200,7 +200,7 @@ fi
 # We really want build-ids. Warn and force generating them if gcc was
 # configure without --enable-linker-build-id
 AC_CACHE_CHECK([whether the compiler generates build-ids], ac_cv_buildid, [dnl
-AC_LINK_IFELSE([AC_LANG_PROGRAM()],[ac_cv_buildid=yes; readelf -n conftest$EXEEXT | grep -q NT_GNU_BUILD_ID || ac_cv_buildid=no],AC_MSG_FAILURE([unexpected compile failure]))])
+AC_LINK_IFELSE([AC_LANG_PROGRAM()],[ac_cv_buildid=yes; $READELF -n conftest$EXEEXT | grep -q NT_GNU_BUILD_ID || ac_cv_buildid=no],AC_MSG_FAILURE([unexpected compile failure]))])
 if test "$ac_cv_buildid" = "no"; then
 	AC_MSG_WARN([compiler doesn't generate build-id by default])
 	LDFLAGS="$LDFLAGS -Wl,--build-id"
-- 
2.27.0