src.opts.mk: Decouple MK_INCLUDES from MK_TOOLCHAIN

Prior to 590461a4b8 installation of include files was controlled
directly by ${MK_TOOLCHAIN}.  590461a4b8 added an INCLUDES knob
defaulting to YES.  Setting WITHOUT_TOOLCHAIN forced it off to retain
existing behaviour.

Decouple them now, as there are reasonable use cases for installing
libraries and include files without a compiler or other tool chain
components.

Reviewed by:	imp, jrtc27
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39918

share/mk/src.opts.mk

@@ -459,7 +459,6 @@ MK_LLD_BOOTSTRAP:= no
 
 .if ${MK_TOOLCHAIN} == "no"
 MK_CLANG:=	no
-MK_INCLUDES:=	no
 MK_LLD:=	no
 MK_LLDB:=	no
 MK_LLVM_BINUTILS:=	no

tools/build/options/WITHOUT_TOOLCHAIN

@@ -1,4 +1,4 @@
 .\" $FreeBSD$
-Do not install header or
+Do not install
 programs used for program development,
 compilers, debuggers etc.