Build system updates (#307)

* build: skip default prefix as '/usr'

Distributions that ship this package will always provide the correct
system prefix, sysconfdir, libdir and such.

As a sane default, the package should use the meson default prefix set
to '/usr/local', which is also the standard in many other build
setups (e.g. autotools...).

* build: print actual full paths in meson output

Do not print the option values, print the full built paths.

meson.build

@@ -21,7 +21,6 @@ project('open5gs', 'c',
     meson_version : '>= 0.43.0',
     default_options : [
         'c_std=gnu89',
-        'prefix=/usr',
     ],
 )
 
@@ -120,11 +119,11 @@ endif
 
 message('\n'.join([
   '',
-  '        prefix:                       ' + get_option('prefix'),
-  '        libdir:                       ' + get_option('libdir'),
-  '        bindir:                       ' + get_option('bindir'),
-  '        sysconfdir:                   ' + get_option('sysconfdir'),
-  '        localstatedir:                ' + get_option('localstatedir'),
+  '        prefix:                       ' + prefix,
+  '        libdir:                       ' + libdir,
+  '        bindir:                       ' + bindir,
+  '        sysconfdir:                   ' + sysconfdir,
+  '        localstatedir:                ' + localstatedir,
   '        source code location:         ' + meson.source_root(),
   '        compiler:                     ' + cc.get_id(),
   '        debugging support:            ' + get_option('buildtype'),