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"

configure.ac

@@ -22,6 +22,11 @@ AM_CONDITIONAL(HAVE_DOXYGEN, test $DOXYGEN != false)
 
 AC_CONFIG_MACRO_DIR([m4])
 
+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 checks for libraries
 PKG_CHECK_MODULES(FFTW3F, fftw3f >= 3.2.0)