[tracker] meson.build: Avoid using meson.source_root() to locate g-ir-merge

[Sam Thursfield <sthursfield src gnome org>]

commit 134be492b98362cbd4793c91a2b05344736c808b
Author: Sam Thursfield <sam thursfield codethink co uk>
Date:   Thu Aug 3 23:28:10 2017 +0100

    meson.build: Avoid using meson.source_root() to locate g-ir-merge
    
    The meson.source_root() variable conflicts with a subprojects feature --
    if Tracker is included as a subproject then source_root() will point to
    the parent project and so we'll fail to find the g-ir-merge script.

 meson.build |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/meson.build b/meson.build
index ed930cf..a27a3dd 100644
--- a/meson.build
+++ b/meson.build
@@ -432,7 +432,7 @@ tracker_writeback_modules_dir = join_paths(get_option('prefix'), get_option('lib
 vapi_dir = join_paths(get_option('prefix'), get_option('datadir'), 'vala', 'vapi')
 
 g_ir_compiler = find_program('g-ir-compiler', gobject_introspection.get_pkgconfig_variable('g_ir_compiler'))
-g_ir_merge = find_program('g-ir-merge', join_paths(meson.source_root(), 'utils', 'g-ir-merge', 'g-ir-merge'))
+g_ir_merge = find_program('g-ir-merge', join_paths(meson.current_source_dir(), 'utils', 'g-ir-merge', 
'g-ir-merge'))
 
 # You can use the gobject_introspection.get_pkgconfig_variable() to find these,
 # but then get_option('prefix') won't be honoured if it differs from the prefix