Check for boost header in sysroot

The check has to detect if boost header is present in the system we are
building for.

Change-Id: I700a11df208c8852ba094d8bff387ad21fa309b2
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

tests/benchmarks/corelib/tools/qregexp/qregexp.pro

@@ -11,7 +11,7 @@ qtHaveModule(script):!pcre {
     QT += script
 }
 
-exists( /usr/include/boost/regex.hpp ){
+exists($$[QT_SYSROOT]/usr/include/boost/regex.hpp) {
 DEFINES+=HAVE_BOOST
 LIBS+=-lboost_regex
 }