Linux: don't explicitly declare -std=gnu99

For a long time Linux has compiled with -std=gnu11, and there are some
parts of the headers that depend on behaviour defined there. Clang is
stricter about enforcing this than GCC, so downgrading to gnu99 can
break the build in some configurations.

Sponsored-by: https://despairlabs.com/sponsor/
Signed-off-by: Rob Norris <[email protected]>

Author: robn

module/Kbuild.in

@@ -2,7 +2,7 @@
 # first.  This ensures its module initialization function is run before
 # any of the other module initialization functions which depend on it.
 
-ZFS_MODULE_CFLAGS += -std=gnu99 -Wno-declaration-after-statement
+ZFS_MODULE_CFLAGS += -Wno-declaration-after-statement
 ZFS_MODULE_CFLAGS += -Wmissing-prototypes
 ZFS_MODULE_CFLAGS += @KERNEL_DEBUG_CFLAGS@ @KERNEL_NO_FORMAT_ZERO_LENGTH@