Restore environment after `make` to avoid mixing-up linker flags (#29984)

Fixes #29983.

scripts/cmake/vcpkg_build_make.cmake

@@ -129,6 +129,8 @@ function(vcpkg_build_make)
             if(LINK_ENV_${cmake_buildtype})
                 set(config_link_backup "$ENV{_LINK_}")
                 set(ENV{_LINK_} "${LINK_ENV_${cmake_buildtype}}")
+            else()
+                unset(config_link_backup)
             endif()
 
             if(arg_ADD_BIN_TO_PATH)
@@ -171,9 +173,8 @@ function(vcpkg_build_make)
                 )
             endif()
 
-            if(config_link_backup)
+            if(DEFINED config_link_backup)
                 set(ENV{_LINK_} "${config_link_backup}")
-                unset(config_link_backup)
             endif()
 
             if(arg_ADD_BIN_TO_PATH)

scripts/cmake/vcpkg_configure_make.cmake

@@ -787,6 +787,8 @@ function(vcpkg_configure_make)
         if(LINK_ENV_${current_buildtype})
             set(link_config_backup "$ENV{_LINK_}")
             set(ENV{_LINK_} "${LINK_ENV_${current_buildtype}}")
+        else()
+            unset(link_config_backup)
         endif()
 
         vcpkg_list(APPEND lib_env_vars LIB LIBPATH LIBRARY_PATH) # LD_LIBRARY_PATH)
@@ -826,9 +828,8 @@ function(vcpkg_configure_make)
         endif()
         z_vcpkg_restore_pkgconfig_path()
         
-        if(link_config_backup)
+        if(DEFINED link_config_backup)
             set(ENV{_LINK_} "${link_config_backup}")
-            unset(link_config_backup)
         endif()
         
         if(arg_ADD_BIN_TO_PATH)