prosody: fix build with musl libc

musl libc does not provide the mallinfo() API. Define WITHOUT_MALLINFO
to skip the code using it.

Fixes:
http://autobuild.buildroot.net/results/42c/42cd6e3be761d1cb358bc50acd7fa58c8aa1c750/
http://autobuild.buildroot.net/results/29d/29d8dcbe013e549a865c0c5953272dab1fe5f777/
http://autobuild.buildroot.net/results/a6d/a6deb7440f80b072432c0f9c048e59abe4edf1c0/

Cc: Dushara Jayasinghe <nidujay@gmail.com>
Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/prosody/prosody.mk

@@ -10,12 +10,15 @@ PROSODY_LICENSE = MIT
 PROSODY_LICENSE_FILES = COPYING
 PROSODY_DEPENDENCIES = host-luainterpreter luainterpreter libidn openssl
 
+PROSODY_CFLAGS = $(TARGET_CFLAGS) -fPIC -std=c99 \
+	$(if BR2_TOOLCHAIN_USES_MUSL,-DWITHOUT_MALLINFO)
+
 PROSODY_CONF_OPTS = \
 	--with-lua-bin=$(HOST_DIR)/usr/bin \
 	--with-lua=$(STAGING_DIR)/usr \
 	--lua-version=$(LUAINTERPRETER_ABIVER) \
 	--c-compiler=$(TARGET_CC) \
-	--cflags="$(TARGET_CFLAGS) -fPIC -std=c99" \
+	--cflags="$(PROSODY_CFLAGS)" \
 	--linker=$(TARGET_CC) \
 	--ldflags="$(TARGET_LDFLAGS) -shared" \
 	--sysconfdir=/etc/prosody \