configure.ac: fix libtool issue  with clang and sanitizer

[this fix already exists in most of the other repos]

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: I577e7d7b07acf76e5d97dcce5da206d10f5e2aeb
---
M configure.ac
1 file changed, 34 insertions(+), 0 deletions(-)

diff --git a/configure.ac b/configure.ac
index 91fb36a..c29012d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -23,6 +23,11 @@
 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

