cmake: standardize USE_WINHTTP

WinHTTP can now be disabled with `USE_WINHTTP=OFF` instead of
`WINHTTP=OFF` to better support the other cmake semantics.

CMakeLists.txt

@@ -93,8 +93,8 @@ endif()
 
 if(WIN32)
 	# By default, libgit2 is built with WinHTTP.  To use the built-in
-	# HTTP transport, invoke CMake with the "-DWINHTTP=OFF" argument.
-	option(WINHTTP             "Use Win32 WinHTTP routines"                         ON)
+	# HTTP transport, invoke CMake with the "-DUSE_WINHTTP=OFF" argument.
+	option(USE_WINHTTP         "Use Win32 WinHTTP routines"                         ON)
 endif()
 
 

cmake/SelectHTTPSBackend.cmake

@@ -19,7 +19,7 @@ IF(USE_HTTPS)
 				MESSAGE(STATUS "Security framework is too old, falling back to OpenSSL")
 				SET(USE_HTTPS "OpenSSL")
 			ENDIF()
-		ELSEIF (WINHTTP)
+		ELSEIF (USE_WINHTTP)
 			SET(USE_HTTPS "WinHTTP")
 		ELSEIF(OPENSSL_FOUND)
 			SET(USE_HTTPS "OpenSSL")

src/CMakeLists.txt

@@ -98,7 +98,7 @@ if(WIN32 AND EMBED_SSH_PATH)
 	set(GIT_SSH 1)
 endif()
 
-if(WIN32 AND WINHTTP)
+if(WIN32 AND USE_WINHTTP)
 	set(GIT_WINHTTP 1)
 
 	# Since MinGW does not come with headers or an import library for winhttp,