Fix finding patch executable when cross-compiling on windows. (#627)

`WIN32` checks whether current build context is windows. This is not correct in case of cross-compiling, because we are interested in finding host's tools, not SDK's tools.

README.md

@@ -80,6 +80,7 @@ On the other hand, if `VERSION` hasn't been explicitly specified, CPM can automa
 `GIT_TAG` can also be set to a specific commit or a branch name such as `master`, however this isn't recommended, as such packages will only be updated when the cache is cleared.
 
 `PATCHES` takes a list of patch files to apply sequentially. For a basic example, see [Highway](examples/highway/CMakeLists.txt).
+We recommend that if you use `PATCHES`, you also set `CPM_SOURCE_CACHE`. See [issue 577](https://github.com/cpm-cmake/CPM.cmake/issues/577).
 
 If an additional optional parameter `EXCLUDE_FROM_ALL` is set to a truthy value, then any targets defined inside the dependency won't be built by default. See the [CMake docs](https://cmake.org/cmake/help/latest/prop_tgt/EXCLUDE_FROM_ALL.html) for details.
 

cmake/CPM.cmake

@@ -490,7 +490,7 @@ function(cpm_add_patches)
 
   # Find the patch program.
   find_program(PATCH_EXECUTABLE patch)
-  if(WIN32 AND NOT PATCH_EXECUTABLE)
+  if(CMAKE_HOST_WIN32 AND NOT PATCH_EXECUTABLE)
     # The Windows git executable is distributed with patch.exe. Find the path to the executable, if
     # it exists, then search `../usr/bin` and `../../usr/bin` for patch.exe.
     find_package(Git QUIET)

examples/boost/CMakeLists.txt

@@ -13,10 +13,12 @@ include(../../cmake/CPM.cmake)
 
 CPMAddPackage(
   NAME Boost
-  VERSION 1.84.0
-  URL https://github.com/boostorg/boost/releases/download/boost-1.84.0/boost-1.84.0.tar.xz
-  URL_HASH SHA256=2e64e5d79a738d0fa6fb546c6e5c2bd28f88d268a2a080546f74e5ff98f29d0e
-  OPTIONS "BOOST_ENABLE_CMAKE ON" "BOOST_INCLUDE_LIBRARIES container\\\;asio" # Note the escapes!
+  VERSION 1.86.0 # Versions less than 1.85.0 may need patches for installation targets.
+  URL https://github.com/boostorg/boost/releases/download/boost-1.86.0/boost-1.86.0-cmake.tar.xz
+  URL_HASH SHA256=2c5ec5edcdff47ff55e27ed9560b0a0b94b07bd07ed9928b476150e16b0efc57
+  OPTIONS "BOOST_ENABLE_CMAKE ON" "BOOST_SKIP_INSTALL_RULES ON" # Set `OFF` for installation
+          "BUILD_SHARED_LIBS OFF" "BOOST_INCLUDE_LIBRARIES container\\\;asio" # Note the escapes!
 )
 
+# `Boost::headers` is also valid
 target_link_libraries(CPMExampleBoost PRIVATE Boost::asio Boost::container)