fix detection of LIBCC for compiler-rt with clang

Maintainer's note: at one point, -lcompiler_rt apparently worked, and
may still work and be preferable if one has manually installed the
library in a public lib directory. but with current versions of clang,
the full pathname to the library file is needed. the original patch
removed the -lcompiler_rt check; I have left it in place in case there
are users depending on it, and since, when it does work, it's
preferable so as not to code a dependency on the specific compiler
version and paths in config.mak.

configure

@@ -598,6 +598,8 @@ tryldflag LDFLAGS_AUTO -Wl,-Bsymbolic-functions
 # Find compiler runtime library
 test -z "$LIBCC" && tryldflag LIBCC -lgcc && tryldflag LIBCC -lgcc_eh
 test -z "$LIBCC" && tryldflag LIBCC -lcompiler_rt
+test -z "$LIBCC" && try_libcc=`$CC -print-libgcc-file-name 2>/dev/null` \
+                 && tryldflag LIBCC "$try_libcc"
 test -z "$LIBCC" && try_libcc=`$CC -print-file-name=libpcc.a 2>/dev/null` \
                  && tryldflag LIBCC "$try_libcc"
 printf "using compiler runtime libraries: %s\n" "$LIBCC"