Create thin archive and convert it afterwards

While this worked fine on rawhide, the MRI script is not accepted
on f37. Apparently + is a special character and something we end
up trying to use libc rather than libc++.a.

Avoid this by creating a thin archive first and then only using
the MRI script to convert it into a fat archive. Weird that there
is apparently no native way to do this.

libcxx.spec

@@ -108,13 +108,16 @@ common_cmake_flags="\
 LIBCXX_A=`find $PWD -name libc++.a`
 mkdir results-static
 pushd results-static
+# Create thin archive
+ar cqT tmp.a $LIBCXX_A %{_libdir}/libc++abi.a
+# Convert into normal archive
 ar -M <<EOM
-CREATE libc++.a
-ADDLIB $LIBCXX_A
-ADDLIB %{_libdir}/libc++abi.a
+CREATE tmp.a
+ADDLIB tmp.a
 SAVE
 END
 EOM
+mv tmp.a libc++.a
 ranlib libc++.a
 popd