Unbundle test dependency dev-cpp/catch and make test compilation optional.

Author: Lucio Sauer <watermanpaint@posteo.net>
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,7 +2,12 @@ cmake_minimum_required(VERSION 3.0.2)
 project(mustache)
 add_library(mustache INTERFACE)
 
+option(TESTS "build tests")
+
 target_compile_features(mustache INTERFACE cxx_std_17)
 target_sources(mustache INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}/mustache.hpp)
 target_include_directories(mustache INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
-add_subdirectory(tests)
\ No newline at end of file
+
+if (TESTS)
+	add_subdirectory(tests)
+endif (TESTS)
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,10 +1,12 @@
+find_package(Catch2 REQUIRED)
+
 add_executable(mustache-unit-tests
     ../mustache.hpp # to show in IDE
-    catch.hpp # to show in IDE
     tests.cpp
 )
 
-target_link_libraries(mustache-unit-tests PRIVATE mustache)
+
+target_link_libraries(mustache-unit-tests PRIVATE mustache Catch2::Catch2)
 
 if (UNIX)
     target_compile_options(mustache-unit-tests PRIVATE -Wall -Wextra -Werror -Wconversion)
--- a/tests/tests.cpp
+++ b/tests/tests.cpp
@@ -29,7 +29,7 @@
 #include "mustache.hpp"
 
 #define CATCH_CONFIG_MAIN
-#include "catch.hpp"
+#include <catch2/catch.hpp>
 
 using namespace kainjow::mustache;