acinclude.m4: Fix "sparse", via detection of GNU make "if" directive.

Make treats tabs very differently from spaces at the beginning of a line,
so this test must use a tab instead of a space.  This partially reverts
commit a0903134d2d60 (acinclude.m4: Expand tabs).

Without this commit, the build system never enables checking with sparse
because it never detects that GNU make "if" works.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>