depend-cleanup: Force a clean build when options change

Similar to the build epoch check, cache a list of source options in the
object tree, and force a clean build if the cached list does not match
the current list, after filtering out options which are known not to
affect the build (e.g. CLEAN, TESTS, WARNS).

This also adds a DEPEND_CLEANUP option (which defaults to yes unless
the CLEAN option is set) which can be used to skip depend-cleanup for
faster incremental builds.

MFC after:	1 week
Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D52011

share/man/man5/src.conf.5

@@ -449,6 +449,14 @@ Clean before building world and/or kernel.
 Note that recording a new epoch in
 .Pa .clean_build_epoch
 in the root of the source tree will also force a clean world build.
+When set, these options are also in effect:
+.Pp
+.Bl -inset -compact
+.It Va WITHOUT_DEPEND_CLEANUP
+(unless
+.Va WITH_DEPEND_CLEANUP
+is set explicitly)
+.El
 .It Va WITHOUT_CPP
 Do not build
 .Xr cpp 1 .
@@ -537,6 +545,12 @@ amd64/amd64, arm64/aarch64, i386/i386, powerpc/powerpc64 and powerpc/powerpc64le
 .It Va WITHOUT_DEBUG_FILES
 Avoid building or installing standalone debug files for each
 executable binary and shared library.
+.It Va WITHOUT_DEPEND_CLEANUP
+Do not attempt to detect if the object tree needs cleaning in part or in
+whole before building.
+This speeds up incremental builds, especially when experimenting with
+build options, but may cause the build to inexplicably fail or produce
+non-functioning binaries.
 .It Va WITH_DETECT_TZ_CHANGES
 Make the time handling code detect changes to the timezone files.
 .It Va WITH_DIALOG

share/mk/src.opts.mk

@@ -86,6 +86,7 @@ __DEFAULT_YES_OPTIONS = \
     CRYPT \
     CUSE \
     CXGBETOOL \
+    DEPEND_CLEANUP \
     DICT \
     DMAGENT \
     DTRACE \
@@ -515,6 +516,10 @@ MK_LLVM_CXXFILT:=	yes
 MK_LOADER_VERIEXEC_PASS_MANIFEST := no
 .endif
 
+.if ${MK_CLEAN} == "yes"
+MK_DEPEND_CLEANUP:=	no
+.endif
+
 #
 # MK_* options whose default value depends on another option.
 #