libsdl2: Fix build when libunwind is not used

clang provides libunwind.h and cmake adds a check to
find libunwind when this header is detected, which was
not the case with automake. The check however is expecting
specific unwinder implementation which provides libunwind-generic
solib, this is not a standard library that all implementations
will provide, therefore make this check optional.

(From OE-Core rev: fb450807774d100b9b568364b014ac46f5642b7e)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-graphics/libsdl2/libsdl2/optional-libunwind-generic.patch

@@ -0,0 +1,18 @@
+Do not error when libunwind-generic is not found, Not all
+implementations of libunwind will provide this library therefore
+do not make it hard error if its not found.
+
+Upstream-Status: Submitted [https://github.com/libsdl-org/SDL/pull/5194]
+Signed-off-by: Khem Raj <raj.khem@gmail.com>
+
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -1358,7 +1358,7 @@ elseif(UNIX AND NOT APPLE AND NOT ANDROI
+       if(HAVE_LIBUNWIND_H)
+         # We've already found the header, so REQUIRE the lib to be present
+         pkg_search_module(UNWIND REQUIRED libunwind)
+-        pkg_search_module(UNWIND_GENERIC REQUIRED libunwind-generic)
++        pkg_search_module(UNWIND_GENERIC libunwind-generic)
+         list(APPEND EXTRA_LIBS ${UNWIND_LIBRARIES} ${UNWIND_GENERIC_LIBRARIES})
+       endif()
+     endif()

meta/recipes-graphics/libsdl2/libsdl2_2.0.18.bb

@@ -18,6 +18,7 @@ PROVIDES = "virtual/libsdl2"
 
 SRC_URI = "http://www.libsdl.org/release/SDL2-${PV}.tar.gz \
            file://0001-Fix-build-against-wayland-1.20.patch \
+           file://optional-libunwind-generic.patch \
 "
 
 S = "${WORKDIR}/SDL2-${PV}"