安装完libpng后，undefined reference to `png_create_write_struct'...解决方法

ubuntu下执行 g++ dgr.cpp，报错：

/tmp/cchTRmbn.o: In function `main':
dgr.cpp:(.text+0x3a): undefined reference to `png_create_write_struct'
dgr.cpp:(.text+0x5d): undefined reference to `png_create_info_struct'
dgr.cpp:(.text+0x7d): undefined reference to `png_destroy_write_struct'
dgr.cpp:(.text+0xb6): undefined reference to `png_destroy_write_struct'
dgr.cpp:(.text+0xd3): undefined reference to `png_init_io'
dgr.cpp:(.text+0x105): undefined reference to `png_set_IHDR'
dgr.cpp:(.text+0x11d): undefined reference to `png_malloc'
dgr.cpp:(.text+0x147): undefined reference to `png_destroy_write_struct'
dgr.cpp:(.text+0x16a): undefined reference to `png_set_PLTE'
dgr.cpp:(.text+0x17d): undefined reference to `png_write_info'
dgr.cpp:(.text+0x189): undefined reference to `png_s