build-sys: meson: Process subdirectories before generating configuration

Subdirectories add to the top-level cdata (specifically, the SIMD
detection happens in the pulsecore meson.build), so we were missing
HAVE_MMX/SSE2/NEON defines without this fix.
diff --git a/meson.build b/meson.build
index 14600db..abe4019 100644
--- a/meson.build
+++ b/meson.build
@@ -674,6 +674,17 @@
 
 check_dep = dependency('check', version : '>= 0.9.10', required : get_option('tests'))
 
+# Subdirs
+
+subdir('po')
+if get_option('man')
+  subdir('man')
+endif
+subdir('shell-completion/bash')
+subdir('shell-completion/zsh')
+subdir('src')
+subdir('vala')
+
 # Now generate config.h from everything above
 configure_file(output : 'config.h', configuration : cdata)
 
@@ -741,17 +752,6 @@
   install_dir : cmakedir,
 )
 
-# Subdirs
-
-subdir('po')
-if get_option('man')
-  subdir('man')
-endif
-subdir('shell-completion/bash')
-subdir('shell-completion/zsh')
-subdir('src')
-subdir('vala')
-
 ############################################################
 
 # Final summary