bootstrap: Fix syntax for Solaris 10 shell (#15317)

In commit v3.1.0-rc2~7^2 (Workaround for short jump tables on PA-RISC,
2014-11-04) we added use of shell syntax not supported on the Solaris
shell.  Avoid using the '!' operator.

Reported-by: Friedrich Haubensak <hsk@imb-jena.de>

bootstrap

@@ -145,7 +145,7 @@ if ${cmake_system_linux}; then
     cmake_machine_parisc=true
   fi
 elif ${cmake_system_hpux}; then
-  if !(uname -m | grep ia64 >/dev/null 2>&1); then
+  if uname -m | grep ia64 >/dev/null 2>&1; then : ; else
     cmake_machine_parisc=true
   fi
 fi