Fixes #404

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

The Curl plugin requires libexpat in order to work these days, so we should
download and build it in order to get the plugin enabled on Windows.

Now that all build failures have been fixed, we can enable the
feature.

configure.ac sets this, but this wasn't used for compiling third-party
libraries.  This setting however is important for libnfs, which adds
fallback definitions for POLLIN and POLLOUT with bogus values.

Workaround for the following LAME build failure:

 error: inlining failed in call to always_inline '_mm_sqrt_ps': target
 specific option mismatch

This is because the LAME build scripts do not check whether SSE is
available; they only check for the presence of the "xmmintrin.h"
header.

Requiring a Pentium 3 CPU is reasonable enough, and it's the first CPU
to feature SSE support.

Enable it in the Windows build script, closes #78.

LAME currently doesn't support Android:

 checking host system type... Invalid configuration `arm-linux-androideabi': system `androideabi' not recognized

Allow multiple arguments.

Fixes various static library linker failures ("bad extended name entry
at header"), because the wrong "ranlib" was used.

Reduce dependencies on global variables.

Got lost in the previous commit.

Not used anymore.

zlib cannot be built out-of-tree, and previously, we were building in
the "src" directory, not in the "build" directory.  The
make_build_path() return value was unused.