[devhelp] build: set warning_level=2 and remove -Wall and -Wextra

[Sébastien Wilmet <swilmet src gnome org>]

commit 068fe73c8491b5b784299148777c19024e331e27
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Fri Nov 22 13:09:42 2019 +0100

    build: set warning_level=2 and remove -Wall and -Wextra
    
    The warning_level=2 adds the -Wall and -Wextra flags (but this is not
    documented in the Meson reference manual).
    
    It permits to silence the following meson warning:
    WARNING: Consider using the builtin warning_level option instead of
    adding warning flags by hand.

 meson.build | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/meson.build b/meson.build
index f6100cea..43d601d3 100644
--- a/meson.build
+++ b/meson.build
@@ -7,6 +7,7 @@ project(
   'devhelp', 'c',
   meson_version : '>= 0.50',
   version : '3.34.0',
+  default_options : ['warning_level=2']
 )
 
 GNOME = import('gnome')
@@ -104,11 +105,10 @@ add_project_arguments(
 # Try to mimic the AX_COMPILER_FLAGS Autotools macro.
 # Some flags are missing when using only the builtin warning_level meson option,
 # even at the maximum level.
+# The following warning_cflags suppose that warning_level=2.
 
 warning_cflags = [
   '-fno-strict-aliasing',
-  '-Wall',
-  '-Wextra',
   '-Wundef',
   '-Wnested-externs',
   '-Wwrite-strings',