autotools: Enable subdir-objects as many plugins use sub-directory ob…

…jects

Many plugins build object files from sub-directories, and when doing
this it is better to enable Automake subdir-objects option.  So, enable
it, as it is good and old enough anyway.

configure.ac

@@ -1,7 +1,7 @@
 AC_PREREQ([2.61])
 dnl Don't forget when updating version number to do GP_CHECK_GEANY also
 AC_INIT([geany-plugins], [1.25])
-AM_INIT_AUTOMAKE([1.8 foreign dist-bzip2 tar-ustar])
+AM_INIT_AUTOMAKE([1.8 foreign dist-bzip2 tar-ustar subdir-objects])
 
 m4_ifdef([AM_SILENT_RULES],[AM_SILENT_RULES([yes])])