MONGOCRYPT-685 fix disabling patching and whitespace (#837)

* set `patch_disabled` to OFF first

Prevents reading a prior value of `patch_disabled` set in the same scope.

* pass `--ignore-whitespace` to `git apply`

To fix observed issue patching libbson on Windows (without Cygwin).

cmake/FetchMongoC.cmake

@@ -6,6 +6,7 @@ set (MONGOC_FETCH_TAG_FOR_LIBBSON "1.27.1" CACHE STRING "The Git tag of mongo-c-
 
 # Add an option to disable patching if a patch command is unavailable.
 option (LIBBSON_PATCH_ENABLED "Whether to apply patches to the libbson library" ON)
+set (patch_disabled OFF)
 if (NOT LIBBSON_PATCH_ENABLED)
     set (patch_disabled ON)
 endif ()

cmake/IntelDFP.cmake

@@ -17,6 +17,7 @@ if (NOT INTEL_DFP_LIBRARY_URL_SHA256 STREQUAL "no-verify")
     set (_hash_arg URL_HASH "${INTEL_DFP_LIBRARY_URL_HASH}")
 endif ()
 
+set (patch_disabled OFF)
 if (NOT INTEL_DFP_LIBRARY_PATCH_ENABLED)
     set (patch_disabled ON)
 endif ()

cmake/Patch.cmake

@@ -31,6 +31,8 @@ function(make_patch_command out)
         if(patch_STRIP_COMPONENTS)
             list(APPEND cmd -p${patch_STRIP_COMPONENTS})
         endif()
+        # Ignore whitespace errors to fix patch errors on Windows: The patch file may be converted to \r\n by git, but libbson fetched with \n.
+        list(APPEND cmd "--ignore-whitespace")
         # git accepts patch filepaths as positional arguments
         list(APPEND cmd ${patch_PATCHES})
     else()