guile: make guile-extension-dir configurable

We use
   guile_dep.get_variable(pkgconfig: 'extensiondir')
but that's not good for everyone.

meson.build

@@ -36,13 +36,13 @@ mandir    = prefixdir / get_option('mandir')
 infodir   = prefixdir / get_option('infodir')
 
 # allow for configuring lispdir, as with autotools.
-# default to <p
 if get_option('lispdir') == ''
   mu4e_lispdir= datadir / join_paths('emacs', 'site-lisp', 'mu4e')
 else
   mu4e_lispdir= get_option('lispdir') / 'mu4e'
 endif
 
+
 ################################################################################
 # compilers / flags
 #
@@ -165,6 +165,16 @@ config_h_data.set_quoted('GZIP_PROGRAM', gzip.full_path())
 
 # soft dependencies
 guile_dep = dependency('guile-3.0', required: get_option('guile'))
+# allow for a custom guile-extension-dir
+if guile_dep.found()
+  custom_guile_xd=get_option('guile-extension-dir')
+  if custom_guile_xd == ''
+    guile_extension_dir = guile_dep.get_variable(pkgconfig: 'extensiondir')
+  else
+    guile_extension_dir = custom_guile_xd
+  endif
+  config_h_data.set_quoted('MU_GUILE_EXTENSION_DIR', guile_extension_dir)
+endif
 
 # emacs -- needed for mu4e compilation
 emacs_name=get_option('emacs')