ROOT-7004: don't include regex header in pch only on mac

this allows the testsuite to pass, to protect against slc6-cc7
compatibility issues but does not fix the naming issue with regex,
the name of which cannot be constructed on mac and is wrong on
linux.

Author: dpiparo

build/unix/makepchinput.sh

@@ -35,6 +35,12 @@ for stlHeader in $stlHeaders; do
     echo '#endif' >> $allheaders
 done
 
+# Special case for regex
+echo "// treat regex separately" >> $allheaders
+echo '#if __has_include("regex") && !defined __APPLE__' >> $allheaders
+echo '#include <regex>' >> $allheaders
+echo '#endif' >> $allheaders
+
 # treat this deprecated headers in a special way
 stlDeprecatedHeaders="strstream"
 echo "// STL Deprecated headers" >> $allheaders