configure: use egrep isntead of grep

grep is used for simple patterns and basic regular expressions (BREs);
egrep can handle extended regular expressions (EREs). A pattern in form
'a|b|c' requires ERE, and BRE isn't enough for non-GNU grep.

configure.ac

@@ -729,7 +729,7 @@ libdovecot_headers=
 libdovecot_c_files=
 non_libdovecot_headers=
 non_libdovecot_c_files=
-all_files=`find $srcdir/src -name '*.[[ch]]' | grep -v '/src/config/all-settings.c' | grep -v '/src/lib-settings/' | grep -v '/test-' | xargs grep '\\(struct setting_parser_info [[a-z]]\\)\\|\\(struct service_settings [[a-z]]\\)\\|\\(<settings checks>\\)' | sed 's/:.*//' | sort | uniq`
+all_files=`find $srcdir/src -name '*.[[ch]]' | grep -v '/src/config/all-settings.c' | grep -v '/src/lib-settings/' | grep -v '/test-' | xargs egrep '(struct setting_parser_info [[a-z]])|(struct service_settings [[a-z]])|(<settings checks>)' | sed 's/:.*//' | sort | uniq`
 for file in $all_files; do
   dir=`echo "$file" | sed 's:/[[^/]]*$::'`
   if echo "$libdovecot_dirs" | grep "$dir" >/dev/null; then