Add automatic OpenSSL & ZLIB detection for cmake

If one is detected, it will automatically link & define the required definition.
This lets its dependencies be applied easily to whatever depends on it.

Note that OpenSSL and ZLIB are still optional, and merely used if found.

Author: sum01

CMakeLists.txt

@@ -1,8 +1,21 @@
 cmake_minimum_required(VERSION 3.7.0 FATAL_ERROR)
 project(httplib VERSION 0.5.12 LANGUAGES CXX)
 
+# Change as needed to set an OpenSSL minimum version.
+# This is used in the installed Cmake config file.
+set(_HTTPLIB_OPENSSL_MIN_VER "1.1.1")
 
-# Include
+# TODO: implement the option of option to correctly split, building, and export with the split.py script.
+# option(HTTPLIB_SPLIT "Uses a Python script to split the header into a header & source file." OFF)
+
+# Threads needed for <thread> on some systems, and for <pthread.h> on Linux
+find_package(Threads REQUIRED)
+# Look quietly since these are optional
+find_package(OpenSSL ${_HTTPLIB_OPENSSL_MIN_VER} QUIET)
+find_package(ZLIB QUIET)
+
+# Used for default, common dirs that the end-user can change (if needed)
+# like CMAKE_INSTALL_INCLUDEDIR or CMAKE_INSTALL_DATADIR
 include(GNUInstallDirs)
 
 add_library(${PROJECT_NAME} INTERFACE)
@@ -12,6 +25,21 @@ target_include_directories(${PROJECT_NAME} INTERFACE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
     $<INSTALL_INTERFACE:include>)
 
+target_link_libraries(${PROJECT_NAME} INTERFACE
+	# Always require threads
+	Threads::Threads
+	# Only link zlib & openssl if they're found
+	$<$<BOOL:${ZLIB_FOUND}>:ZLIB::ZLIB>
+	$<$<BOOL:${OPENSSL_FOUND}>:OpenSSL::SSL OpenSSL::Crypto>
+)
+
+# Auto-define the optional support if those packages were found
+target_compile_definitions(${PROJECT_NAME} INTERFACE
+	$<$<BOOL:${ZLIB_FOUND}>:CPPHTTPLIB_ZLIB_SUPPORT>
+	$<$<BOOL:${OPENSSL_FOUND}>:CPPHTTPLIB_OPENSSL_SUPPORT>
+)
+
+
 install(TARGETS ${PROJECT_NAME} EXPORT httplibConfig
     ARCHIVE  DESTINATION ${CMAKE_INSTALL_LIBDIR}
     LIBRARY  DESTINATION ${CMAKE_INSTALL_LIBDIR}