Make xbyak.py installation location arch-independent

I was investigating an issue with the Fedora package build:
builds on i686 would yield a package with /usr/lib/pkgconfig/xbyak.pc,
while builds on amd64 would yield a package with /usr/lib64/pkgconfig/xbyak.pc.

xbyak is arch-indepdent, in the sense of the installed payload being identical
on all architectures and located in a non-arch-specific directory (/usr/inlude).
The .pc file should be arch-independent too. Right now, if we install the
package from a different architecture, we would find the files in /usr/include,
but not the .pc file. (E.g. on amd64 the pkg-config search path is
/usr/lib64/pkgconfig:/usr/share/pkgconfig, so /usr/lib/pkgconfig/xbyak.pc will
not be found.) So install the file to $prefix/pkgconfig (usually
/usr/share/pkgconfig) so it will be found properly.

We need to specify 'includedir' in the variable list for pkgconfig.generate(),
because by default meson does not include this variable when dataonly:true
is used. The $prefix/include pattern is evaluated in meson, to handle the
case where the user specified an include_dir as absolute path.

meson.build

@@ -10,7 +10,8 @@ project(
 	default_options: 'b_ndebug=if-release'
 )
 
-install_subdir('xbyak', install_dir: get_option('includedir'))
+include_dir = get_option('prefix') / get_option('includedir')
+install_subdir('xbyak', install_dir: include_dir)
 
 xbyak_dep = declare_dependency(include_directories: include_directories('.'))
 
@@ -22,7 +23,9 @@ import('pkgconfig').generate(
 	name: meson.project_name(),
 	description: 'JIT assembler for x86(IA32), x64(AMD64, x86-64)',
 	version: meson.project_version(),
-	url: 'https://github.com/herumi/xbyak'
+	url: 'https://github.com/herumi/xbyak',
+	variables: ['includedir=@0@'.format(include_dir)],
+	dataonly: true,
 )
 
 if meson.version().version_compare('>=0.50.0')