grep: fix build failure for musl

musl doesn't implement re_match and friends and grep fails to
detect this properly, so force internal regex routines for musl.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/grep/grep.mk

@@ -9,7 +9,8 @@ GREP_SITE = $(BR2_GNU_MIRROR)/grep
 GREP_SOURCE = grep-$(GREP_VERSION).tar.xz
 GREP_LICENSE = GPLv3+
 GREP_LICENSE_FILES = COPYING
-GREP_CONF_OPTS = --disable-perl-regexp --without-included-regex
+GREP_CONF_OPTS = --disable-perl-regexp \
+	$(if $(BR2_TOOLCHAIN_USES_MUSL),--with-included-regex)
 GREP_DEPENDENCIES = $(if $(BR2_NEEDS_GETTEXT_IF_LOCALE),gettext)
 
 # link with iconv if enabled