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.

test/integration/templates/using-patch-adder/lists.in.cmake

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+project(using-patch-adder)
+
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+
+include("%{cpm_path}")
+
+%{packages}
+
+add_executable(using-patch-adder using-patch-adder.cpp)
+
+target_link_libraries(using-patch-adder adder)

test/integration/templates/using-patch-adder/patches/001-test_patches_command.patch

@@ -0,0 +1,12 @@
+diff --git a/code/adder/adder.hpp b/code/adder/adder.hpp
+index fdb9324..7c2fa00 100644
+--- a/code/adder/adder.hpp
++++ b/code/adder/adder.hpp
+@@ -1,6 +1,6 @@
+ #pragma once
+ 
+-namespace adder
++namespace patched
+ {
+ int add(int a, int b);
+ }

test/integration/templates/using-patch-adder/patches/002-test_patches_command.patch

@@ -0,0 +1,12 @@
+diff --git a/code/adder/adder.cpp b/code/adder/adder.cpp
+index fc6e767..44b1197 100644
+--- a/code/adder/adder.cpp
++++ b/code/adder/adder.cpp
+@@ -1,6 +1,6 @@
+ #include "adder.hpp"
+ 
+-namespace adder
++namespace patched
+ {
+ int add(int a, int b)
+ {

test/integration/templates/using-patch-adder/using-patch-adder.cpp

@@ -0,0 +1,8 @@
+#include <adder/adder.hpp>
+#include <cstdio>
+
+int main() {
+  int sum = patched::add(5, 3);
+  std::printf("%d\n", sum);
+  return 0;
+}