python/build/meson.py: set BOOST_ROOT for Meson 0.54

NEWS

@@ -4,6 +4,8 @@ ver 0.21.25 (not yet released)
 * input
   - file: detect premature end of file
   - smbclient: don't send credentials to MPD clients
+* Windows/Android:
+  - fix Boost detection after breaking change in Meson 0.54
 
 ver 0.21.24 (2020/06/10)
 * protocol

python/build/meson.py

@@ -91,7 +91,12 @@ def configure(toolchain, src, build, args=()):
         '--cross-file', cross_file,
     ] + args
 
-    subprocess.check_call(configure, env=toolchain.env)
+    env = toolchain.env.copy()
+
+    # Meson 0.54 requires the BOOST_ROOT environment variable
+    env['BOOST_ROOT'] = toolchain.install_prefix
+
+    subprocess.check_call(configure, env=env)
 
 class MesonProject(Project):
     def __init__(self, url, md5, installed, configure_args=[],