meson: support definition of a custom abi tag

this is intended to allow for distributions to have alternate streams, such as
glibc-x86_64 or darwin-aarch64

meson_options.txt

@@ -4,3 +4,4 @@ option('compressed-help', description: 'Compress help database, needs lua-zlib',
 option('lua', description: 'Build luaapk (lua bindings)', type: 'feature', value: 'auto')
 option('lua_version', description: 'Lua version to build against', type: 'string', value: '5.3')
 option('static_apk', description: 'Also build apk.static', type: 'boolean', value: false)
+option('abi_tag', description: 'Define a custom ABI tag for default arch', type: 'string')

src/meson.build

@@ -115,6 +115,11 @@ apk_cargs = [
 	'-D_ATFILE_SOURCE',
 ]
 
+apk_abi_tag = get_option('abi_tag')
+if apk_abi_tag != ''
+	apk_cargs += ['-DAPK_ABI_TAG="@0@"'.format(apk_abi_tag)]
+endif
+
 libapk_shared = shared_library(
 	'apk',
 	libapk_src,