meson: bump minimum version to 0.51

We were already using some features from 0.49:
 WARNING: Project specifies a minimum meson_version '>= 0.47.2' but uses features which were added in newer versions:
 * 0.48.0: {'meson.add_dist_script'}
 * 0.49.0: {'Calling "add_dist_script" with multiple arguments'}

Debian 10 uses meson 0.49.2, but it will get out of support in 2 months
so we can start considering it as a too old version. Next oldest meson
version used by the distros that we follow is Ubuntu 20.04 with meson
0.53.2.

Raise to 0.51 as it is supported by all the distros that we test (except
Debian 10) and it contains all the features that we need for the next
commits.

meson.build

@@ -13,7 +13,7 @@ project(
     'c_std=gnu11',
     'warning_level=2' # value "2" will add "-Wall" and "-Wextra" to the compiler flags
   ],
-  meson_version: '>= 0.47.2',
+  meson_version: '>= 0.51.0',
 )
 
 nm_name = meson.project_name()