[gnome-disk-utility] build: Document custom build switches

[Kai Lüke <kailueke src gnome org>]

commit 77e3a6cacdf523a39cca24f43d25eceb8cf6c213
Author: Kai Lüke <kailueke riseup net>
Date:   Tue Nov 14 21:55:24 2017 +0900

    build: Document custom build switches
    
    The build option usage specific to GNOME Disks
    may not be discoverable for people new to meson.
    
    Show the option usage when printing their current
    value during the build configuration.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=790303

 INSTALL     |   12 ++++++++++++
 meson.build |    3 ++-
 2 files changed, 14 insertions(+), 1 deletions(-)
---
diff --git a/INSTALL b/INSTALL
index ba94202..5304de9 100644
--- a/INSTALL
+++ b/INSTALL
@@ -5,6 +5,17 @@ http://mesonbuild.com/Quick-guide.html
 configure the build directory:
   meson builddir
 
+(skip if not needed:
+configure the build directory with options:
+  meson -D libsystemd=true|false -D gsd_plugin=true|false builddir
+
+list the available options and their current values:
+  meson configure builddir
+
+reconfigure a value:
+  meson configure builddir -D libsystemd=true|false -D gsd_plugin=true|false
+)
+
 compile:
   cd builddir
   ninja
@@ -13,6 +24,7 @@ run GNOME Disks:
   builddir/src/disks/gnome-disks
 
 produce a release tarball:
+  cd builddir
   ninja dist
 
 This will build against the system libraries.
diff --git a/meson.build b/meson.build
index 41f8cfe..fd62de1 100644
--- a/meson.build
+++ b/meson.build
@@ -149,5 +149,6 @@ output += '        sysconfdir:                 ' + gdu_sysconfdir + '\n\n'
 output += '        Use libsystemd:             ' + enable_libsystemd.to_string() + '\n'
 output += '        Build g-s-d plug-in:        ' + enable_gsd_plugin.to_string() + '\n\n'
 output += '        compiler:                   ' + cc.get_id() + '\n'
-output += '        cflags:                     ' + ' '.join(compiler_flags) + '\n'
+output += '        cflags:                     ' + ' '.join(compiler_flags) + '\n\n'
+output += '        (Change with: meson configure BUILDDIR -D libsystemd=true|false -D 
gsd_plugin=true|false)\n\n'
 message(output)