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: I999adf84a34c03765ce6c32ece0e61d0ac6e1c13
diff --git a/configure.ac b/configure.ac
index 48de07a..d693c67 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,6 +26,11 @@
 AC_PROG_INSTALL
 LT_INIT
 
+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