Revert "Use system version of boost regex, when available"

This reverts commit eaa342ca.

CMakeLists.txt

@@ -135,16 +135,8 @@ if(GXX_LIBSTDCPP)
 	if(STD_REGEX_RUNNING STREQUAL FAILED_TO_RUN)
 		message(STATUS "You are probably trying to compile using the g++ standard library which contains a crashing std::regex implementation. Will try to use boost::regex instead")
 
-		set(Boost_USE_STATIC_LIBS ON)
-		find_package(Boost 1.72 COMPONENTS regex QUIET)
-
-		if(Boost_FOUND)
-			message(STATUS "Using already installed boost::regex")
-			list(APPEND CIFPP_REQUIRED_LIBRARIES Boost::regex)
-		else()
-			set(BOOST_REGEX_STANDALONE ON)
-			add_git_submodule(regex EXCLUDE_FROM_ALL)
-		endif()
+		set(BOOST_REGEX_STANDALONE ON)
+		add_git_submodule(regex EXCLUDE_FROM_ALL)
 	endif()
 endif()