Improve meson build.

-Add all required packages as wrap dependencies
-Move source lsitnings to separate files
-Proper dependencies declarations
-Set global D compiler arguments
-Add few asserts
-Add test exe
-Other small improvements

Tested with dmd with betterC, demos executable has to be run from 'demos' directory to properly load assets.

subprojects/packagefiles/bindbc-loader/meson.build

@@ -0,0 +1,24 @@
+project('bindbc-loader', 'd', version : '0.3.2', default_options: ['default_library=static'])
+
+# Files
+src = files(
+	'source/bindbc/loader/package.d',
+	'source/bindbc/loader/sharedlib.d',
+	'source/bindbc/loader/system.d',
+)
+
+inc = include_directories('source')
+
+# Dependencies
+lib = library('bindbc-loader', src,
+	include_directories : [inc],
+	pic : true,
+	d_module_versions: ['BindBC_Static'],
+)
+
+bindbc_loader_dep = declare_dependency(
+	include_directories : [inc],
+	link_with : lib,
+)
+
+meson.override_dependency('bindbc-loader', bindbc_loader_dep)