package/libkrb5: fix GCC 15.x build issues

Backport upstream commit 4b9d7f7c107f01a61600fddcd8cde3812d0366a2
would allow to solve GCC 15.x issues, but it's a 313 KB patch. So
instead, let's build with -std=gnu99. It requires a bit of
reorganization on how CFLAGS are handled.

Fixes:

  http://autobuild.buildroot.net/results/4460089f5989277e6143f6e3d66f71371913ee4b/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 7d0456d510090ee90634486702a73c75102e99f1)
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/libkrb5/libkrb5.mk

@@ -16,6 +16,8 @@ LIBKRB5_CPE_ID_PRODUCT = kerberos_5
 LIBKRB5_DEPENDENCIES = host-bison $(TARGET_NLS_DEPENDENCIES)
 LIBKRB5_INSTALL_STAGING = YES
 
+LIBKRB5_CFLAGS = $(TARGET_CFLAGS) -std=gnu99
+
 # The configure script uses AC_TRY_RUN tests to check for those values,
 # which doesn't work in a cross-compilation scenario. Therefore,
 # we feed the configure script with the correct answer for those tests
@@ -38,9 +40,11 @@ LIBKRB5_CONF_OPTS = \
 # Error: selected processor does not support `mcr p15,0,r2,c7,c10,5' in Thumb mode
 # so, we deactivate Thumb mode
 ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
-LIBKRB5_CONF_ENV += CFLAGS="$(TARGET_CFLAGS) -marm"
+LIBKRB5_CFLAGS += -marm
 endif
 
+LIBKRB5_CONF_ENV += CFLAGS="$(LIBKRB5_CFLAGS)"
+
 # Enabling static and shared at the same time is not supported
 ifeq ($(BR2_SHARED_STATIC_LIBS),y)
 LIBKRB5_CONF_OPTS += --disable-static