Add automake subdir-options for modules

The automake project is apparently changing behavior in their next
major version and warning everyone who relies on subdir-options to add
it explicitly.

build/modmake.rulesam

+AUTOMAKE_OPTIONS = foreign subdir-objects
 AM_CFLAGS   = $(SWITCH_AM_CFLAGS) $(SWITCH_ANSI_CFLAGS)
 AM_CPPFLAGS = $(SWITCH_AM_CXXFLAGS)
 AM_LDFLAGS  = $(SWITCH_AM_LDFLAGS)

src/mod/applications/mod_curl/Makefile.am

 include $(top_srcdir)/build/modmake.rulesam
 
-AUTOMAKE_OPTIONS = foreign
+AUTOMAKE_OPTIONS += foreign
 MODNAME=mod_curl
 
 JSON_C=json-c-0.9

src/mod/languages/mod_v8/Makefile.am

 include $(top_srcdir)/build/modmake.rulesam
 
-AUTOMAKE_OPTIONS = foreign
+AUTOMAKE_OPTIONS += foreign
 
 # V8 version to use
 V8=v8-3.24.14