- Allow WITH_CCACHE_BUILD to work with boost.

This speeds up building from 35 minutes to 5 minutes.

  If you are building boost with ccache by changing CC="ccache cc",
  change to using WITH_CCACHE_BUILD=yes in make.conf. This will
  automatically handle installing and using ccache for port building,
  while avoiding build breakages.

Reported by:	Samu Nuutamo
Discussed with:	bapt (who marked it unsafe)

devel/boost-all/compiled.mk

@@ -20,7 +20,6 @@ BJAM_ARGS+=	cxxflags=-fPIC
 # of gcc/g++):
 BJAM_ARGS+=    cxxflags="${CXXFLAGS:N-O*}" cflags="${CFLAGS:N-O*}"
 
-NO_CCACHE=	yes
 .if ${CC:T:Mgcc*}
 BOOST_TOOLSET:=	gcc
 .elif ${CC:T:Mclang}