libcgroup: fix passing CFLAGS/LDFLAGS to configure

LIBCGROUP_CONF_ENV is a string used to pass environment variables to the
confgiure script, '+=' operator doesn't have any special meaning inside
it, so CFLAGS+=... is passed to shell, overwriting previous CFLAGS
value. Replace CFLAGS+="..." with CFLAGS="$(TARGET_CFLAGS) ...".

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libcgroup/libcgroup.mk

@@ -16,8 +16,8 @@ LIBCGROUP_INSTALL_STAGING = YES
 # large file support. See https://bugzilla.redhat.com/show_bug.cgi?id=574992
 # for more information.
 LIBCGROUP_CONF_ENV = \
-	CXXFLAGS+="-U_FILE_OFFSET_BITS" \
-	CFLAGS+="-U_FILE_OFFSET_BITS"
+	CXXFLAGS="$(TARGET_CXXFLAGS) -U_FILE_OFFSET_BITS" \
+	CFLAGS="$(TARGET_CFLAGS) -U_FILE_OFFSET_BITS"
 
 LIBCGROUP_CONF_OPT = \
 	--disable-tools \