CPM Patch step incorrectly assumes CMake external command is executed in shell, fails to apply multiple patches listed in `PATCHES`

[christopherbate]

In CI environments, whenever my CPMAddPackage command has PATCHES set with multiple files, I get an error

/usr/bin/patch: '&&': extra operand
/usr/bin/patch: Try '/usr/bin/patch --help' for more information.

This occurs because in function cpm_add_patches, the patch command is built by concatenating a bunch of strings like

patch -p1 < "$first_patch_file" && patch -p1 < "$second_patch_file"

And then these strings are appended to the PATCH_COMMAND argument for hand-off to a downstream function (FetchContent?).

This assumes that the PATCH_COMMAND executor is a shell that understands things like &&, which apparently is not always the case.

After looking at CMake docs, I hacked together a basic workaround which appears to work. CMake apparently allows issuing multiple commands for the PATCH_COMMAND as long as you declare COMMAND for each subsequent command after the first:

PATCH_COMMAND
  patch -p1 -i $first_patch
  COMMAND patch -p1 -i $second_aptch
```