Add support for Meson build system

Tested on:
 - Linux/x86* with gcc
 - Android armv7 arm64 x86 x86_64 with clang
 - Windows x86 x86_64 with Visual Studio 2017
 - Windows x86 x86_64 with MinGW
 - macOS x86_64 with clang
 - iOS arm64 x86_64 with clang

Co-authored by: Nirbheek Chauhan <nirbheek@centricular.com>

https://gitlab.xiph.org/xiph/opus/-/merge_requests/13

meson/read-sources-list.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+#
+# opus/read-sources-list.py
+#
+# Parses .mk files and extracts list of source files.
+# Prints one line per source file list, with filenames space-separated.
+
+import sys
+
+if len(sys.argv) < 2:
+  sys.exit('Usage: {} sources_foo.mk [sources_bar.mk...]'.format(sys.argv[0]))
+
+for input_fn in sys.argv[1:]:
+  with open(input_fn, 'r', encoding='utf8') as f:
+    text = f.read()
+    text = text.replace('\\\n', '')
+
+    # Remove empty lines
+    lines = [line for line in text.split('\n') if line.strip()]
+
+    # Print SOURCES_XYZ = file1.c file2.c
+    for line in lines:
+      values = line.strip().split('=', maxsplit=2)
+      if len(values) != 2:
+        raise RuntimeError('Unable to parse line "{}" from file "{}"'.format(line, input_fn))
+      var, files = values
+      sources_list = [f for f in files.split(' ') if f]
+      print(var.strip(), '=', ' '.join(sources_list))