Mesonize JPEG Quant Smooth

Now SIMD works on amd64, although the build remains questionable,
because it assumes that all of its compiler flags will work.

This way we lose an uncomfortable git submodule.

Also, add Meson subprojects to .gitignore.

fiv-io.c

@@ -32,10 +32,7 @@
 #include <stdio.h>
 
 #include <jpeglib.h>
-// This library is tricky to build, simply make it work at all.
-#define NO_SIMD
-#include <jpeg-quantsmooth/quantsmooth.h>
-#undef NO_SIMD
+#include <libjpegqs.h>
 #endif  // HAVE_JPEG_QS
 
 // Colour management must be handled before RGB conversions.