btrfs-progs: build: make libudev selectable

Previously the build would be optional based on what is installed on the
system. Add an option to make this selectable by user, ie. to allow
build without libudev even if it is installed on the system.

For most users and distros libudev dependency should be ok, but we've
had requests to disable various features (embedded, systems with
non-systemd user space base system) so let's do the same.

Signed-off-by: David Sterba <dsterba@suse.com>

INSTALL

@@ -22,6 +22,12 @@ dependencies are not desired.
 - libsodium
 - libkcapi
 
+Optionally, multipath device detection requires libudev and running udev
+daemon, as it's the only source of the path information. Static build has a
+fallback and does not need static version of libudev.
+
+- libudev
+
 For zoned device support, the system headers installed in /usr/include/linux
 must be 5.10 or newer.
 

configure.ac

@@ -317,13 +317,14 @@ fi
 AS_IF([test "x$enable_zstd" = xyes], [BTRFSRESTORE_ZSTD=1], [BTRFSRESTORE_ZSTD=0])
 AC_SUBST(BTRFSRESTORE_ZSTD)
 
-PKG_CHECK_EXISTS([libudev], [pkg_config_libudev=yes], [pkg_config_libudev=no])
-if test "x$pkg_config_libudev" = xyes; then
+AC_ARG_ENABLE([libudev],
+      AS_HELP_STRING([--disable-libudev], [build without libudev support (for multipath)]),
+      [], [enable_libudev=yes]
+)
+
+if test "x$enable_libudev" = xyes; then
 	PKG_CHECK_MODULES([LIBUDEV], [libudev])
 	AC_DEFINE([HAVE_LIBUDEV], [1], [Define to 1 if libudev is available])
-else
-	AC_MSG_CHECKING([for LIBUDEV])
-	AC_MSG_RESULT([no])
 fi
 
 AC_ARG_ENABLE([python],
@@ -430,6 +431,7 @@ AC_MSG_RESULT([
 	Python bindings:    ${enable_python}
 	Python interpreter: ${PYTHON}
 	crypto provider:    ${cryptoprovider} ${cryptoproviderversion}
+	libudev:            ${enable_libudev}
 	zoned device:       ${enable_zoned}
 	${experimental_msg}
 	Type 'make' to compile.