Adding PATCHES keyword. (#558)

* Adding PATCHES keyword.

* Formatting fix.

* Move cpm_add_patches() outside if/else scopes.

* cmake-format: add PATCHES to CPMAddPackage.

* Integration tests for PATCHES command.

* Use get_filename_component() in place of cmake_path() for use with all cmake versions 3.14 and above.

* Added an example and improved comment for cpm_add_patches.

examples/highway/main.cpp

@@ -0,0 +1,27 @@
+#include <hwy/contrib/sort/vqsort.h>  // hwy::VQSort() for large data sets
+
+#include <cstdint>
+#include <random>
+#include <vector>
+
+// Use hwy::VQSort to sort larger vectors
+inline void sort_large(std::vector<double>& v) {
+  hwy::VQSort(v.data(), v.size(), hwy::SortAscending{});
+}
+
+int main(int, char**) {
+  std::random_device random_device;
+  std::default_random_engine random_engine(random_device());
+  std::uniform_real_distribution<double> uniform_dist(0.0, 100.0);
+
+  const std::size_t sz = 100000;
+  std::vector<double> v;
+  v.reserve(sz);
+  for (std::size_t i = 0; i < sz; ++i) {
+    v.push_back(uniform_dist(random_engine));
+  }
+
+  sort_large(v);
+
+  return 0;
+}