Lower minimum meson version back to 0.45.0

Note that some configurations (e.g. an emscripten build) will definitely
fail on this version. The minimum recommended version is 0.48.0, and a
warning will be printed if using something older.

README.rst

@@ -34,7 +34,7 @@ Build-only dependencies
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 -  Python >= 3.5
--  meson >= 0.46.0 (build system; >=0.48.0 recommended)
+-  meson >= 0.45.0 (build system; >=0.48.0 recommended)
 
 Optional:
 

meson.build

@@ -1,7 +1,7 @@
 project('taisei', 'c',
     license : 'MIT',
     version : 'v1.3-dev',
-    meson_version : '>=0.46.0',
+    meson_version : '>=0.45.0',
     default_options : [
         'c_std=c11',
 
@@ -13,6 +13,12 @@ project('taisei', 'c',
     ]
 )
 
+minimum_recommended_meson_version = '0.48.0'
+
+if meson.version().version_compare('<@0@'.format(minimum_recommended_meson_version))
+    warning('Old Meson version detected. Try upgrading to at least @0@ if the build fails.'.format(minimum_recommended_meson_version))
+endif
+
 cc = meson.get_compiler('c')
 python3 = import('python3').find_python()
 macos_app_bundle = get_option('macos_bundle') and host_machine.system() == 'darwin'