build: remove stdlib.h from header checks
build: remove stdlib.h from header checks

We already use a mix of <cstdlib> and stdlib.h unconditionally throughout
the codebase.

Us checking this header also duplicates work already done by autotools.
Currently stdlib.h is checked for 3 times during a ./configure run, after
this change, at least it's only twice.

lateminer/bitcoin,namecoin/namecoin-core,ajtowns/bitcoin,fanquake/bitcoin,tecnovert/particl-core,particl/particl-core,sstone/bitcoin,kallewoof/bitcoin,bitcoinsSG/bitcoin,bitcoinsSG/bitcoin,AkioNak/bitcoin,lateminer/bitcoin,ajtowns/bitcoin,kallewoof/bitcoin,kallewoof/bitcoin,namecoin/namecoin-core,sstone/bitcoin,particl/particl-core,bitcoinsSG/bitcoin,namecoin/namecoin-core,lateminer/bitcoin,particl/particl-core,bitcoin/bitcoin,AkioNak/bitcoin,sstone/bitcoin,bitcoin/bitcoin,mruddy/bitcoin,sipsorcery/bitcoin,AkioNak/bitcoin,sstone/bitcoin,tecnovert/particl-core,mruddy/bitcoin,bitcoin/bitcoin,sstone/bitcoin,bitcoinsSG/bitcoin,tecnovert/particl-core,bitcoin/bitcoin,AkioNak/bitcoin,bitcoin/bitcoin,ajtowns/bitcoin,AkioNak/bitcoin,sipsorcery/bitcoin,sipsorcery/bitcoin,namecoin/namecoin-core,particl/particl-core,lateminer/bitcoin,namecoin/namecoin-core,tecnovert/particl-core,jambolo/bitcoin,fanquake/bitcoin,jambolo/bitcoin,lateminer/bitcoin,bitcoinsSG/bitcoin,bitcoinsSG/bitcoin,bitcoin/bitcoin,mruddy/bitcoin,sipsorcery/bitcoin,AkioNak/bitcoin,sipsorcery/bitcoin,namecoin/namecoin-core,lateminer/bitcoin,ajtowns/bitcoin,jambolo/bitcoin,particl/particl-core,fanquake/bitcoin,jambolo/bitcoin,jambolo/bitcoin,jambolo/bitcoin,kallewoof/bitcoin,fanquake/bitcoin,fanquake/bitcoin,tecnovert/particl-core,sipsorcery/bitcoin,particl/particl-core,ajtowns/bitcoin,mruddy/bitcoin,kallewoof/bitcoin,ajtowns/bitcoin,tecnovert/particl-core,mruddy/bitcoin,sstone/bitcoin,fanquake/bitcoin,kallewoof/bitcoin,mruddy/bitcoin