Introduce an EXPERIMENTAL option for both src.conf(5) and the kernel.

In the last decade(s) we have seen both short term or long term projects
committed to the tree which were considered or even marked "experimental".
While out-of-tree development has become easier than it used to be in
CVS times, there still is a need to have the code shipping with HEAD but
not enabled by default.

While people may think about VIMAGE as one of the recent larger, long term
projects, early protocol implementations (before they are standardised)
are others.  (Free)BSD historically was one of the operating systems
which would have running code at early stages and help develop and
influence standardisation and the industry.

Give developers an opportunity to be more pro-active for early adoption
or running large scale code changes stumbling over each others but not
the user's feet.  I have not added the option to NOTES in order to avoid
breaking supported option builds, which require constant compile testing.

Discussed with:	people in the corridor

4 files changed, 7 insertions, 1 deletions

diff --git a/share/man/man5/src.conf.5 b/share/man/man5/src.conf.5
index 37fb5ef571c1..c62a6d605615 100644
--- a/share/man/man5/src.conf.5
+++ b/share/man/man5/src.conf.5
@@ -1,6 +1,6 @@
 .\" DO NOT EDIT-- this file is @generated by tools/build/options/makeman.
 .\" $FreeBSD$
-.Dd October 25, 2018
+.Dd October 30, 2018
 .Dt SRC.CONF 5
 .Os
 .Sh NAME
@@ -659,6 +659,8 @@ An alternate bootstrap tool chain must be provided.
 .It Va WITHOUT_EXAMPLES
 Set to avoid installing examples to
 .Pa /usr/share/examples/ .
+.It Va WITH_EXPERIMENTAL
+Set to include experimental features in the build.
 .It Va WITH_EXTRA_TCP_STACKS
 Set to build extra TCP stack modules.
 .It Va WITHOUT_FDT
diff --git a/share/mk/src.opts.mk b/share/mk/src.opts.mk
index 7091db36459f..10aa1e8986ac 100644
--- a/share/mk/src.opts.mk
+++ b/share/mk/src.opts.mk
@@ -197,6 +197,7 @@ __DEFAULT_NO_OPTIONS = \
     BSD_GREP \
     CLANG_EXTRAS \
     DTRACE_TESTS \
+    EXPERIMENTAL \
     GNU_GREP_COMPAT \
     HESIOD \
     LIBSOFT \
diff --git a/sys/conf/options b/sys/conf/options
index 2519d17411e7..9d86cbdfd663 100644
--- a/sys/conf/options
+++ b/sys/conf/options
@@ -95,6 +95,7 @@ _COMPAT_LINUX32	opt_compat.h	# XXX: make sure opt_compat.h exists
 COMPILING_LINT	opt_global.h
 CY_PCI_FASTINTR
 DEADLKRES	opt_watchdog.h
+EXPERIMENTAL	opt_global.h
 EXT_RESOURCES	opt_global.h
 DIRECTIO
 FILEMON		opt_dontuse.h
diff --git a/tools/build/options/WITH_EXPERIMENTAL b/tools/build/options/WITH_EXPERIMENTAL
new file mode 100644
index 000000000000..677edb6a081a
--- /dev/null
+++ b/tools/build/options/WITH_EXPERIMENTAL
@@ -0,0 +1,2 @@
+.\" $FreeBSD$
+Set to include experimental features in the build.