package/libidn2: fix build with libunistring

Fix the following build failure with libunistring raised since the
addition of the package in commit
ffb85a4a16df94e4980057a194e5fdc3c8332e32:

/home/autobuild/autobuild/instance-2/output-1/per-package/libidn2/host/bin/../lib/gcc/aarch64-buildroot-linux-gnu/11.3.0/../../../../aarch64-buildroot-linux-gnu/bin/ld: warning: libunistring.so.2, needed by ../lib/.libs/libidn2.so, not found (try using -rpath or -rpath-link)
/home/autobuild/autobuild/instance-2/output-1/per-package/libidn2/host/bin/../lib/gcc/aarch64-buildroot-linux-gnu/11.3.0/../../../../aarch64-buildroot-linux-gnu/bin/ld: ../lib/.libs/libidn2.so: undefined reference to `u8_strconv_to_encoding'

[...]

aarch64-buildroot-linux-gnu-gcc: ERROR: unsafe header/library path used in cross-compilation: '-L/usr/lib'

Fixes:
 - http://autobuild.buildroot.org/results/30ac50512cd4b4cb3ecc97514a72d1f316a1b33a
 - http://autobuild.buildroot.org/results/c225ff4ef007b9a3ca56e6b601687aaa33699675

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit a9e5b0255d47babf45f1f6e13dccf65bbbc9f84c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libidn2/libidn2.mk

@@ -13,10 +13,12 @@ LIBIDN2_DEPENDENCIES = \
 	host-pkgconf \
 	$(TARGET_NLS_DEPENDENCIES) \
 	$(if $(BR2_PACKAGE_LIBICONV),libiconv)
+LIBIDN2_CONF_OPTS = --without-included-libunistring
 LIBIDN2_INSTALL_STAGING = YES
 
 ifeq ($(BR2_PACKAGE_LIBUNISTRING),y)
 LIBIDN2_DEPENDENCIES += libunistring
+LIBIDN2_CONF_OPTS += --with-libunistring-prefix=$(STAGING_DIR)/usr/lib
 endif
 
 ifeq ($(BR2_PACKAGE_LIBIDN2_BINARY),)