configure.ac: fix bashism in ARM optimization handling

Breaks configure when /bin/sh isn't bash with:

configure: Trying to force-enable ARMv6 media instructions...
checking if assembler supports ARMv6 media instructions on ARM... yes
configure: Trying to force-enable NEON instructions...
checking if assembler supports NEON instructions on ARM... yes
./configure.lineno: 12799: Bad substitution

Fix it by using the %% expansion to remove everything from the first
space instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>

configure.ac

@@ -333,9 +333,9 @@ AS_IF([test x"${enable_asm}" = x"yes"],[
 
 AM_CONDITIONAL([CPU_ARM], [test "$cpu_arm" = "yes"])
 AM_CONDITIONAL([OPUS_ARM_INLINE_ASM],
-    [test x"${inline_optimization:0:3}" = x"ARM"])
+    [test x"${inline_optimization%% *}" = x"ARM"])
 AM_CONDITIONAL([OPUS_ARM_EXTERNAL_ASM],
-    [test x"${asm_optimization:0:3}" = x"ARM"])
+    [test x"${asm_optimization%% *}" = x"ARM"])
 
 AS_IF([test x"$enable_rtcd" = x"yes"],[
     AS_IF([test x"$rtcd_support" != x"no"],[