package/re2: fix build with gcc <= 5

Fix the following build failure with gcc <= 5 raised since bump to
version 2022-02-01 in commit d2cf91e0c6a082261cc7ebb06dfdbd7bc2c2956f
and
https://github.com/google/re2/commit/a022cc0c55b0519629d64d775e7a5195af34a477

./re2/sparse_array.h:266:37: warning: defaulted and deleted functions only available with -std=c++11 or -std=gnu++11
 SparseArray<Value>::SparseArray() = default;
                                     ^
./re2/sparse_array.h: In copy constructor 're2::SparseArray<Value>::SparseArray(const re2::SparseArray<Value>&)':
./re2/sparse_array.h:273:3: error: 'copy_n' is not a member of 'std'
   std::copy_n(src.sparse_.data(), src.max_size(), sparse_.data());
   ^

Fixes:
 - http://autobuild.buildroot.org/results/8ebbac67f32fe24416a1f72aad0afd42b567b403

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/re2/re2.mk

@@ -10,19 +10,23 @@ RE2_LICENSE = BSD-3-Clause
 RE2_LICENSE_FILES = LICENSE
 RE2_INSTALL_STAGING = YES
 
+RE2_MAKE_OPTS = \
+	$(TARGET_CONFIGURE_OPTS) \
+	CXXFLAGS="$(TARGET_CXXFLAGS) -std=c++11"
+
 define RE2_BUILD_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) \
+	$(TARGET_MAKE_ENV) $(MAKE) $(RE2_MAKE_OPTS) \
 		-C $(@D) $(if $(BR2_STATIC_LIBS),static)
 endef
 
 define RE2_INSTALL_STAGING_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) \
+	$(TARGET_MAKE_ENV) $(MAKE) $(RE2_MAKE_OPTS) \
 		DESTDIR="$(STAGING_DIR)" prefix=/usr -C $(@D) \
 		$(if $(BR2_STATIC_LIBS),static-install,install)
 endef
 
 define RE2_INSTALL_TARGET_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) \
+	$(TARGET_MAKE_ENV) $(MAKE) $(RE2_MAKE_OPTS) \
 		DESTDIR="$(TARGET_DIR)" prefix=/usr -C $(@D) \
 		$(if $(BR2_STATIC_LIBS),static-install,install)
 endef