[gimp] configure.ac: fix `--with-linux-input` handling with upcoming autoconf-2.70

[Jehan <jehanp src gnome org>]

commit cebeb90a87105cd6e35bcb357d53cc04c828ca21
Author: Sergei Trofimovich <slyfox gentoo org>
Date:   Sun Oct 25 18:09:21 2020 +0000

    configure.ac: fix `--with-linux-input` handling with upcoming autoconf-2.70
    
    Upcoming autoconf-2.70 exposes deficiency in configure.ac:
    
    ```
    $ autoconf-2.70_beta2 && ./configure --host=x86_64-pc-linux-gnu
    ./configure: line 1430: 5: Bad file descriptor
    checking whether  is declared... ./configure: line 1432: ${+y}: bad
    ```
    
    It happens because macros are called with parameters using insufficient quoting.
    
    More details at https://lists.gnu.org/archive/html/bug-autoconf/2020-10/msg00027.html
    
    The fix only amends `--with-linux-input`. Other cases of underquoting
    will need to be handled separately.
    
    Fix-by: Zack Weinberg
    Signed-off-by: Sergei Trofimovich <slyfox gentoo org>

 configure.ac | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 96312f706e..63b85be07a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2135,15 +2135,14 @@ fi
 AC_ARG_WITH(linux-input, [  --without-linux-input   don't build linux input event controller module])
 
 have_linux_input="no (linux input support disabled)"
-if test "x$with_linux_input" != "xno"; then
-  AC_CHECK_HEADER(linux/input.h,
-       AC_CHECK_DECL(KEY_OK,
-               have_linux_input=yes,
-               have_linux_input="no (needs Linux 2.6)",
-               [#include <linux/input.h>]))
-fi
-
-AM_CONDITIONAL(HAVE_LINUX_INPUT, test "x$have_linux_input" = xyes)
+AS_IF([test "x$with_linux_input" != "xno"],
+  [AC_CHECK_HEADER([linux/input.h],
+    [AC_CHECK_DECL([KEY_OK],
+                   [have_linux_input=yes],
+                   [have_linux_input="no (needs Linux 2.6)"],
+                   [#include <linux/input.h>])])])
+
+AM_CONDITIONAL([HAVE_LINUX_INPUT], [test "x$have_linux_input" = xyes])
 
 
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