build: Switch to using AS_IF in configure.ac rather than shell script ‘if’
build: Switch to using AS_IF in configure.ac rather than shell script ‘if’

This is necessary because using certain autoconf macros inside a shell
script ‘if’ can cause problems if the macros perform a one-time
initialisation. The initialisation variables will be lost if the ‘if’ branch
isn’t taken.

See: https://bugzilla.gnome.org/show_bug.cgi?id=682346
See: https://bugzilla.gnome.org/show_bug.cgi?id=681413

GNOME/folks,renatofilho/folks2,renatofilho/folks,renatofilho/folks,renatofilho/folks2,pwithnall/folks,GNOME/folks,pwithnall/folks,renatofilho/folks,renatofilho/folks2,renatofilho/folks2,GNOME/folks,renatofilho/folks,pwithnall/folks