Fix build error on GCC 10 caused by alloca bound warnings

Since GCC 10 warnings for unbounded alloca calls are turned on by
default. The size of the allocation comes from the java side of the
bindings and thus appears to be unchecked on the native side.

CHANGES.md

@@ -12,7 +12,7 @@ Features
 
 Bug Fixes
 ---------
-
+* [#1244](https://github.com/java-native-access/jna/issues/1244): Fix building on GCC 10 - [@matthiasblaesing](https://github.com/matthiasblaesing).
 
 Release 5.6.0
 =============

native/Makefile

@@ -321,14 +321,20 @@ endif
 endif
 
 # CC_OPTS only applied to objects build for jnidispatch, not for libffi
-# -Wno-unknown-warning-option 
+# -Wno-unknown-warning-option
 #                => Suppress warning for unknown warnings
 # -Werror        => Treat warnings as errors
 # -Wno-clobbered => Silence GCC warning about clobbered automatic variables.
 #                   The "longjmp" case only happens in protect.h in the logic
 #                   that implements "protected" mode. In that case an exception
 #                   is raised and the value of the potentially clobbered
 #                   variables is ignored.
+# -Wno-alloca-larger-than => Silence warnings about unbounded alloca calls in
+#                   the dispatch logic. GCC tries to assert, that the size of
+#                   the allocated memory is bounded and thus a check for the
+#                   variable needs to be inplace. For the JNA case, the size
+#                   comes from the Java side, so checks are not visible on the
+#                   C side and thus can't be checked there.
 #
 # Enable this only on GCC versions, that can work with it. It is assumed, that
 # GCC version 4.X is the lower bound. That version is problematic, as it
@@ -337,7 +343,7 @@ ifeq ($(CC),gcc)
     GCC_MAJOR_VERSION = $(shell gcc -dumpversion | cut -f 1 -d '.')
     ifneq ($(GCC_MAJOR_VERSION),4)
 	ifneq ($(GCC_MAJOR_VERSION),3)
-	    LOC_CC_OPTS=-Wno-unknown-warning-option -Werror -Wno-clobbered -Wno-unused-variable
+	    LOC_CC_OPTS=-Wno-unknown-warning-option -Werror -Wno-clobbered -Wno-unused-variable -Wno-alloca-larger-than
 	endif
     endif
 else