src.sys.mk: Support src.conf in SRCTOP

If SRCCONF is not defined and src.conf exists at the top level of the
source tree, use that instead of /etc/src.conf.

MFC after:	3 days
Reviewed by:	kevans, imp
Differential Revision:	https://reviews.freebsd.org/D52470

Author: dag-erling

.gitignore

@@ -29,3 +29,4 @@ tags
 .clangd
 .ccls-cache
 sys/*/compile
+/src.conf

share/man/man5/src.conf.5

@@ -1,5 +1,5 @@
 .\" DO NOT EDIT-- this file is @generated by tools/build/options/makeman.
-.Dd August 20, 2025
+.Dd September 10, 2025
 .Dt SRC.CONF 5
 .Os
 .Sh NAME
@@ -29,8 +29,11 @@ variables that control the aspects of how the system builds.
 .Pp
 The default location of
 .Nm
-is
-.Pa /etc/src.conf ,
+is the top level of the source tree, or
+.Pa /etc/src.conf
+if no
+.Nm
+is found in the source tree itself,
 though an alternative location can be specified in the
 .Xr make 1
 variable

share/mk/src.sys.mk

@@ -6,7 +6,11 @@
 
 .if !defined(_WITHOUT_SRCCONF)
 # Allow user to configure things that only effect src tree builds.
+.if exists(${SRCTOP}/src.conf)
+SRCCONF?=	${SRCTOP}/src.conf
+.else
 SRCCONF?=	/etc/src.conf
+.endif
 .if !empty(SRCCONF) && \
     (exists(${SRCCONF}) || ${SRCCONF} != "/etc/src.conf") && \
     !target(_srcconf_included_)

sys/conf/kern.pre.mk

@@ -8,7 +8,11 @@
 # the rest of /usr/src, but they still always process SRCCONF even though
 # the normal mechanisms to prevent that (compiling out of tree) won't
 # work. To ensure they do work, we have to duplicate thee few lines here.
+.if exists(${SRCTOP}/src.conf)
+SRCCONF?=	${SRCTOP}/src.conf
+.else
 SRCCONF?=	/etc/src.conf
+.endif
 .if (exists(${SRCCONF}) || ${SRCCONF} != "/etc/src.conf") && !target(_srcconf_included_)
 .include "${SRCCONF}"
 _srcconf_included_:

tools/build/options/makeman

@@ -222,8 +222,11 @@ variables that control the aspects of how the system builds.
 .Pp
 The default location of
 .Nm
-is
-.Pa /etc/src.conf ,
+is the top level of the source tree, or
+.Pa /etc/src.conf
+if no
+.Nm
+is found in the source tree itself,
 though an alternative location can be specified in the
 .Xr make 1
 variable