grub-mount: Add support for libfuse3

The libfuse 3.0.0 got released in 2016, with some API changes compared to 2.x.
This commit introduces support for 3.x while keeping it compatible with 2.6
as a fallback still.

To detect fuse3, switch configure over to use pkg-config, which is simpler yet
more reliable than looking for library and header manually. Also set
FUSE_USE_VERSION that way, as it depends on the used libfuse version.

Now that the CFLAGS are read from pkg-config, use just <fuse.h>, which works
with 2.x as well as 3.x and is recommended by libfuse upstream.

One behavior change of libfuse3 is that FUSE_ATOMIC_O_TRUNC is set by default,
which means that open with O_TRUNC is passed as-is instead of calling the
truncate operation. With libfuse2, truncate failed with -ENOSYS and that was
returned to the application. To make O_TRUNC fail with libfuse3, return -EROFS
explicitly if writing was requested.

Signed-off-by: Fabian Vogt <fvogt@suse.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

configure.ac

@@ -1797,17 +1797,11 @@ if test x"$enable_grub_mount" = xno ; then
 fi
 
 if test x"$grub_mount_excuse" = x ; then
-  AC_CHECK_LIB([fuse], [fuse_main_real], [],
-               [grub_mount_excuse="need FUSE library"])
-fi
-
-if test x"$grub_mount_excuse" = x ; then
-  # Check for fuse headers.
-  SAVED_CPPFLAGS="$CPPFLAGS"
-  CPPFLAGS="$CPPFLAGS -DFUSE_USE_VERSION=26"
-  AC_CHECK_HEADERS([fuse/fuse.h], [],
-  	[grub_mount_excuse=["need FUSE headers"]])
-  CPPFLAGS="$SAVED_CPPFLAGS"
+  PKG_CHECK_MODULES([FUSE], [fuse3], [FUSE_CFLAGS="$FUSE_CFLAGS -DFUSE_USE_VERSION=32"], [
+    PKG_CHECK_MODULES([FUSE], [fuse], [FUSE_CFLAGS="$FUSE_CFLAGS -DFUSE_USE_VERSION=26"], [
+      grub_mount_excuse="need fuse or fuse3 libraries"
+    ])
+  ])
 fi
 
 if test x"$enable_grub_mount" = xyes && test x"$grub_mount_excuse" != x ; then