Merge pull request #3660 from mstrap/mingw

MinGW builds should optionally create DLLs without "lib" prefix

CMakeLists.txt

@@ -608,6 +608,8 @@ IF (SONAME)
 	IF (LIBGIT2_FILENAME)
 		ADD_DEFINITIONS(-DLIBGIT2_FILENAME=\"${LIBGIT2_FILENAME}\")
 		SET_TARGET_PROPERTIES(git2 PROPERTIES OUTPUT_NAME ${LIBGIT2_FILENAME})
+	ELSEIF (DEFINED LIBGIT2_PREFIX)
+		SET_TARGET_PROPERTIES(git2 PROPERTIES PREFIX "${LIBGIT2_PREFIX}")
 	ENDIF()
 ENDIF()
 STRING(REPLACE ";" " " LIBGIT2_PC_LIBS "${LIBGIT2_PC_LIBS}")