Add abstract frontend to HTTP support and stop using curl on Windows

misc/nsis/Makefile

@@ -19,9 +19,6 @@ endif
 ifndef USE_OPENAL_DLOPEN
 USE_OPENAL_DLOPEN=1
 endif
-ifndef USE_CURL_DLOPEN
-USE_CURL_DLOPEN=0
-endif
 ifndef USE_INTERNAL_ZLIB
 USE_INTERNAL_ZLIB=1
 endif
@@ -50,9 +47,6 @@ endif
 ifeq ($(USE_OPENAL_DLOPEN),1)
 	DEFINES+= -DUSE_OPENAL_DLOPEN
 endif
-ifeq ($(USE_CURL_DLOPEN),1)
-	DEFINES+= -DUSE_CURL_DLOPEN
-endif
 ifeq ($(USE_INTERNAL_ZLIB),1)
 	DEFINES+= -DUSE_INTERNAL_ZLIB
 endif