package/pkg-cmake: add CMAKE_SYSTEM_VERSION to toolchainfile.cmake

Quoting the CMake documentation:

  When the CMAKE_SYSTEM_NAME variable is set explicitly to enable cross
  compiling then the value of CMAKE_SYSTEM_VERSION must also be set
  explicitly to specify the target system version.

Thus, we should also set CMAKE_SYSTEM_VERSION in toolchainfile.cmake. It
is supposed to be set to the value of `uname -r` on the target. We don't
have that exact value available (unless we build the kernel), but the
value of BR2_TOOLCHAIN_HEADERS_AT_LEAST contains the (minimum) version
of the kernel it will run on, so it should be OK for all practical
purposes.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 36568732e40923d19274f1c997e278a1c4e6c99e)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-cmake.mk

@@ -264,6 +264,7 @@ $(HOST_DIR)/share/buildroot/toolchainfile.cmake:
 		-e 's#@@TARGET_CC@@#$(subst $(HOST_DIR)/,,$(call qstrip,$(TARGET_CC)))#' \
 		-e 's#@@TARGET_CXX@@#$(subst $(HOST_DIR)/,,$(call qstrip,$(TARGET_CXX)))#' \
 		-e 's#@@TARGET_FC@@#$(subst $(HOST_DIR)/,,$(call qstrip,$(TARGET_FC)))#' \
+		-e 's#@@CMAKE_SYSTEM_VERSION@@#$(call qstrip,$(BR2_TOOLCHAIN_HEADERS_AT_LEAST))#' \
 		-e 's#@@CMAKE_SYSTEM_PROCESSOR@@#$(call qstrip,$(CMAKE_SYSTEM_PROCESSOR))#' \
 		-e 's#@@TOOLCHAIN_HAS_FORTRAN@@#$(if $(BR2_TOOLCHAIN_HAS_FORTRAN),1,0)#' \
 		-e 's#@@CMAKE_BUILD_TYPE@@#$(if $(BR2_ENABLE_DEBUG),Debug,Release)#' \

support/misc/toolchainfile.cmake.in

@@ -15,6 +15,7 @@ string(REPLACE "/share/buildroot" "" RELOCATED_HOST_DIR ${CMAKE_CURRENT_LIST_DIR
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR})
 
 set(CMAKE_SYSTEM_NAME Buildroot)
+set(CMAKE_SYSTEM_VERSION @@CMAKE_SYSTEM_VERSION@@)
 set(CMAKE_SYSTEM_PROCESSOR @@CMAKE_SYSTEM_PROCESSOR@@)
 
 # Set the {C,CXX}FLAGS appended by CMake depending on the build type