configure: sanitizer builds (--enable-asan/tsan/ubsan) are built with LTO; --enable-debug is not #4861

What

--enable-asan, --enable-tsan and --enable-ubsan all build with -flto:

$ ./configure --enable-asan --without-tcmalloc
  Sanitizer enabled    : asan
  Building with LTO    : yes
$ grep '^GF_CFLAGS' libglusterfs/src/Makefile
  GF_CFLAGS = ... -fsanitize=address -fno-omit-frame-pointer ... -flto

--enable-debug is exempted from LTO on purpose (0ed6489bd2, "configure: disable
LTO when building with debug", #1967). The sanitizer builds are diagnostic builds of
the same kind and were simply never covered: a sanitizer build stays out of LTO only
when --enable-debug happens to be given as well.

Why

The LTO gate keys only on BUILD_DEBUG:

configure.ac: AS_IF([ test "x$BUILD_DEBUG" = "xno" && test "x$enable_lto" != "xno" ], [ ... -flto ... ])

The sanitizer options set SANITIZER (asan/tsan/ubsan) but never BUILD_DEBUG, so
the gate still enables LTO for them.

Why it matters

A sanitizer build exists to be read, not shipped. LTO adds a whole-program
optimisation pass and a slow link step to a build that is already slow, and one more
variable to a report that should be read literally. Keeping the sanitizer knob and the
LTO knob orthogonal is the decision #1967 already took for --enable-debug.

Fix

Also require SANITIZER = none in the gate (one line; accompanying PR). Verified on
devel 2326b1da07 with gcc 16.2.1: with the fix --enable-asan, --enable-tsan and
--enable-ubsan each report Building with LTO : no, while a plain ./configure is
unchanged (Building with LTO : yes).