configure: Give a reason for disabled features
configure: Give a reason for disabled features

This adds a short reason to the summary at the end of configure (for options
where it makes sense, e.g. not debug or ipv6).

Example:

  charset:      no (icu-uc not found via pkg-config)

Signed-off-by: Uli Schlachter <346c0ad49ac8427b61b3b55abbe37dc0ebd524f2@znc.in>

BtbN/znc,ollie27/znc,dgw/znc,YourBNC/znc,dgw/znc,reedloden/znc,wolfy1339/znc,Kriechi/znc,Adam-/znc,Kriechi/nobnc,markusj/znc,markusj/znc,badloop/znc,Phansa/znc,Phansa/znc,DarthGandalf/znc,Phansa/znc,jreese/znc,Mikaela/znc,DarthGandalf/znc,badloop/znc,Kriechi/znc,badloop/znc,kashike/znc,elyscape/znc,YourBNC/znc,BtbN/znc,Phansa/znc,Hasimir/znc,jreese/znc,DarthGandalf/znc,Adam-/znc,Zarthus/znc,aarondunlap/znc,KiNgMaR/znc,kashike/znc,GLolol/znc,GLolol/znc,znc/znc,elyscape/znc,aarondunlap/znc,jpnurmi/znc,TuffLuck/znc,Adam-/znc,Mkaysi/znc,KielBNC/znc,znc/znc,KielBNC/znc,kashike/znc,ollie27/znc,Kriechi/znc,Mikaela/znc,withinsoft/znc,elyscape/znc,jpnurmi/znc,psychon/znc,markusj/znc,wolfy1339/znc,Zarthus/znc,wolfy1339/znc,badloop/znc,BtbN/znc,Adam-/znc,Mkaysi/znc,anthonyryan1/znc,Kriechi/znc,wolfy1339/znc,jpnurmi/znc,reedloden/znc,TuffLuck/znc,withinsoft/znc,jreese/znc,Mikaela/znc,GLolol/znc,YourBNC/znc,Zarthus/znc,GLolol/znc,anthonyryan1/znc,Mikaela/znc,markusj/znc,badloop/znc,evilnet/znc,dgw/znc,Zarthus/znc,GLolol/znc,psychon/znc,psychon/znc,elyscape/znc,znc/znc,ollie27/znc,aarondunlap/znc,aarondunlap/znc,Mkaysi/znc,KiNgMaR/znc,ollie27/znc,anthonyryan1/znc,dgw/znc,evilnet/znc,Adam-/znc,Mkaysi/znc,GLolol/znc,DarthGandalf/znc,withinsoft/znc,Hasimir/znc,anthonyryan1/znc,KiNgMaR/znc,dgw/znc,reedloden/znc,withinsoft/znc,DarthGandalf/znc,KielBNC/znc,Zarthus/znc,badloop/znc,kashike/znc,wolfy1339/znc,kashike/znc,withinsoft/znc,Kriechi/nobnc,KielBNC/znc,Mkaysi/znc,Phansa/znc,jpnurmi/znc,Mikaela/znc,znc/znc,Hasimir/znc,psychon/znc,ollie27/znc,Hasimir/znc,Mikaela/znc,KiNgMaR/znc,reedloden/znc,Kriechi/znc,YourBNC/znc,dgw/znc,YourBNC/znc,Mkaysi/znc,reedloden/znc,evilnet/znc,znc/znc,evilnet/znc,BtbN/znc,evilnet/znc,znc/znc,reedloden/znc,Adam-/znc,wolfy1339/znc,Hasimir/znc,Kriechi/nobnc,jpnurmi/znc,TuffLuck/znc,jpnurmi/znc,Kriechi/znc,BtbN/znc,Phansa/znc,anthonyryan1/znc,psychon/znc,elyscape/znc,KielBNC/znc,TuffLuck/znc,kashike/znc,KiNgMaR/znc,YourBNC/znc,KielBNC/znc,KiNgMaR/znc,Mikaela/znc,aarondunlap/znc,TuffLuck/znc,jreese/znc,anthonyryan1/znc,DarthGandalf/znc,Hasimir/znc,Mkaysi/znc,withinsoft/znc,evilnet/znc,BtbN/znc,badloop/znc,ollie27/znc,jpnurmi/znc,markusj/znc,jreese/znc,TuffLuck/znc,Hasimir/znc,Adam-/znc,aarondunlap/znc,anthonyryan1/znc,TuffLuck/znc,psychon/znc,Zarthus/znc,aarondunlap/znc,elyscape/znc,markusj/znc,jreese/znc