Allow enable_vspipe and enable_python_module without building a wheel.

Upstream's meson.build assumes "build_wheel=false" means a packaging
build where vspipe and the Python module are not wanted, and silently
overrides the user-supplied -Denable_vspipe / -Denable_python_module
back to false.  For a Gentoo system install we want everything --
shared libraries, headers, pkgconfig, the Python extension module, and
the vspipe binary -- without producing a wheel.  Drop the override so
the explicit -D options are honoured.

The install_subdir('include', ...) call inside the python module block
is only meaningful for wheels (it bundles the C-API headers into the
wheel root), so guard it with `if build_wheel` -- otherwise meson dies
because install_dir = [] is not a valid string for install_subdir().
For non-wheel builds the headers are already installed via
install_headers() under the earlier `if not build_wheel` block.

--- a/meson.build
+++ b/meson.build
@@ -14,11 +14,6 @@
 enable_python_module = get_option('enable_python_module')
 build_wheel = get_option('build_wheel')

-if not build_wheel
-    enable_vspipe = false
-    enable_python_module = false
-endif
-
 cxx = meson.get_compiler('cpp')

 if cxx.get_argument_syntax() == 'gcc'
@@ -267,12 +262,14 @@ if enable_core
             subdir: 'vapoursynth',
         )

-        install_subdir(
-            'include',
-            exclude_directories: 'cython',
-            exclude_files: ['VapourSynth.h', 'VSHelper.h'],
-            install_dir: install_dir,
-        )
+        if build_wheel
+            install_subdir(
+                'include',
+                exclude_directories: 'cython',
+                exclude_files: ['VapourSynth.h', 'VSHelper.h'],
+                install_dir: install_dir,
+            )
+        endif
     endif

     if build_wheel and cxx.get_argument_syntax() == 'msvc'