configure.ac: fix libtool issue with clang and sanitizer

As pointed out at https://github.com/libexpat/libexpat/issues/312
libtool does not play nice with clang sanitizer builds at all.
For those builds LD shoud be set to clang too (and LDFLAGS needs the
sanitizer flags as well), because the clang compiler driver knows how
linking to the sanitizer libs works, but then at a later stage libtool
fails to actually produce the shared libraries and the build fails. This
is fixed by this patch.

Addtionally LD_LIBRARY_PATH has no effect on conftest runs during
configure time, so the rpath needs to be set to the asan library path to
ensure the configure run does not fail due to a missing asan library,
i.e.:

SANS='-fsanitize=memory -fsanitize-recover=all -shared-libsan'
export CC=clang-10
ASANPATH=$(dirname `$CC -print-file-name=libclang_rt.asan-x86_64.so`)
export LDFLAGS="-Wl,-rpath,$ASANPATH $SANS $LDFLAGS"

Change-Id: I74cdca5c8bd5a32b6fb05906280cc1cdd64e9369

configure.ac

@@ -26,6 +26,11 @@ AC_PROG_MKDIR_P
 AC_PROG_CC
 AC_PROG_INSTALL
 
+dnl patching ${archive_cmds} to affect generation of file "libtool" to fix linking with clang 
+AS_CASE(["$LD"],[*clang*],
+  [AS_CASE(["${host_os}"],
+     [*linux*],[archive_cmds='$CC -shared $pic_flag $libobjs $deplibs $compiler_flags $wl-soname $wl$soname -o $lib'])])
+
 dnl check for pkg-config (explained in detail in libosmocore/configure.ac)
 AC_PATH_PROG(PKG_CONFIG_INSTALLED, pkg-config, no)
 if test "x$PKG_CONFIG_INSTALLED" = "xno"; then