Tentatively try to unbreak the build on arm* architectures. There were
two kinds of problems:

1. Sloppy handling of 8-bit data and assuming that `char' is signed by

default in AdPlug and Commodore 64 SID plugins code;

2. Incorrect detection of SSE2 support. Contrary to GCC, Clang always

accepts `-msse2' command line option, even on !x86; it just tells that
it won't be used. As a result, SSE2 code was compiled whilst it could
not be, causing the build to fail.

While here, remove no longer relevant patching of the configure script.