Move zfs_gitrev.h to build directory

Currently an out-of-tree build does not work with read-only source
directory because zfs_gitrev.h can't be created. Move this file to the
build directory, which is more appropriate for a generated file, and
drop the dist-hook for zfs_gitrev.h. There is no need to distribute this
file since it will be regenerated as part of the compilation in any
case.

scripts/make_gitrev.sh tries to avoid updating zfs_gitrev.h if there has
been no change, however this doesn't cover the case when the source
directory is not in git: in that case zfs_gitrev.h gets overwritten even
though it's always "unknown". Simplify the logic to always write out a
new version of zfs_gitrev.h, compare against the old and overwrite only
if different. This is now simple enough to just include in the
Makefile, so drop the script.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10493

module/Kbuild.in

@@ -22,6 +22,7 @@ ZFS_MODULE_CFLAGS += @KERNEL_DEBUG_CFLAGS@  @NO_FORMAT_ZERO_LENGTH@
 ifneq ($(KBUILD_EXTMOD),)
 zfs_include = @abs_top_srcdir@/include
 ZFS_MODULE_CFLAGS += -include @abs_top_builddir@/zfs_config.h
+ZFS_MODULE_CFLAGS += -I@abs_top_builddir@/include
 else
 zfs_include = $(srctree)/include/zfs
 ZFS_MODULE_CFLAGS += -include $(zfs_include)/zfs_config.h

module/Makefile.bsd

@@ -4,7 +4,7 @@ WITH_CTF=1
 
 .include <bsd.sys.mk>
 
-SRCDIR= ${.CURDIR}
+SRCDIR=${.CURDIR}
 INCDIR=${.CURDIR:H}/include
 
 KMOD=	openzfs
@@ -19,6 +19,7 @@ KMOD=	openzfs
 	${SRCDIR}/zfs
 
 
+CFLAGS+= -I${.OBJDIR:H}/include
 CFLAGS+= -I${INCDIR}
 CFLAGS+= -I${INCDIR}/spl
 CFLAGS+= -I${INCDIR}/os/freebsd