Fix -D build flags containing escaped quotes (#645)

Fixes #638

Done by removed unneeded quotes from cmake's add_compiler_definitions()

CMakeLists.txt

@@ -236,9 +236,9 @@ target_link_libraries(${PROJECT_NAME} ${_INTERFACE_OR_PUBLIC}
 
 # Set the definitions to enable optional features
 target_compile_definitions(${PROJECT_NAME} ${_INTERFACE_OR_PUBLIC}
-	$<$<BOOL:${HTTPLIB_IS_USING_BROTLI}>:"CPPHTTPLIB_BROTLI_SUPPORT">
-	$<$<BOOL:${HTTPLIB_IS_USING_ZLIB}>:"CPPHTTPLIB_ZLIB_SUPPORT">
-	$<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:"CPPHTTPLIB_OPENSSL_SUPPORT">
+	$<$<BOOL:${HTTPLIB_IS_USING_BROTLI}>:CPPHTTPLIB_BROTLI_SUPPORT>
+	$<$<BOOL:${HTTPLIB_IS_USING_ZLIB}>:CPPHTTPLIB_ZLIB_SUPPORT>
+	$<$<BOOL:${HTTPLIB_IS_USING_OPENSSL}>:CPPHTTPLIB_OPENSSL_SUPPORT>
 )
 
 # Cmake's find_package search path is different based on the system