build: Switch CLEAN back on by default

There have been too many issues with non-META_MODE incremental builds
recently, and it is clear that most users, even developers, were not
aware that the default had been switched.

This will be revisited once more work has been done to help prevent
future breakage.

This reverts commit ba373fca78.

Reviewed by:	markj, jhb
Differential Revision:	https://reviews.freebsd.org/D51828

UPDATING

@@ -27,6 +27,10 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 15.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20250808:
+	The CLEAN option has been switched back from default-off to
+	default-on.  This reverts the 20240729 change below.
+
 20250807:
 	routed(8) and route6d(8) have moved to the FreeBSD-rip package.
 	If you use either of these, you should install the new package.

share/man/man5/src.conf.5

@@ -1,5 +1,5 @@
 .\" DO NOT EDIT-- this file is @generated by tools/build/options/makeman.
-.Dd August 4, 2025
+.Dd August 8, 2025
 .Dt SRC.CONF 5
 .Os
 .Sh NAME
@@ -411,8 +411,8 @@ Build clang-format.
 .It Va WITHOUT_CLANG_FULL
 Avoid building the ARCMigrate, Rewriter and StaticAnalyzer components of
 the Clang C/C++ compiler.
-.It Va WITH_CLEAN
-Clean before building world and/or kernel.
+.It Va WITHOUT_CLEAN
+Do not clean before building world and/or kernel.
 .It Va WITHOUT_CPP
 Do not build
 .Xr cpp 1 .

share/mk/src.opts.mk

@@ -80,6 +80,7 @@ __DEFAULT_YES_OPTIONS = \
     CDDL \
     CLANG \
     CLANG_BOOTSTRAP \
+    CLEAN \
     CPP \
     CROSS_COMPILER \
     CRYPT \
@@ -200,7 +201,6 @@ __DEFAULT_NO_OPTIONS = \
     BHYVE_SNAPSHOT \
     CLANG_EXTRAS \
     CLANG_FORMAT \
-    CLEAN \
     DIALOG \
     DETECT_TZ_CHANGES \
     DISK_IMAGE_TOOLS_BOOTSTRAP \

tools/build/options/WITH_CLEAN

@@ -1 +0,0 @@
-Clean before building world and/or kernel.