［help！！！］安装cario时出了问题，说是requested PNG backend could not be enabled
./configure时出现下面的错误：
checking for cairo's PNG backend...
configure: WARNING: Could not find libpng in the pkg-config search path
checking whether cairo's PNG backend could be enabled... no
configure: error: requested PNG backend could not be enabled
然后我输入# whereis libpng.pc
libpng: /usr/lib/libpng.so /usr/lib/libpng12.a /usr/lib/libpng12.so /usr/lib/libpng.a /usr/include/libpng /usr/share/man/man3/libpng.3.gz
看起来，我的系统中是有libpng库的
但我输入 # pkg-config --modversion libpng
Package libpng was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpng.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libpng' found
pkg-config却找不到，这个怎么解决呀？
[解决办法]
Perhaps you should add the directory containing `libpng.pc'
to the PKG_CONFIG_PATH environment variable
看看这句话 说不定配置文件中默认的PKG_CONFIG_PATH和你系统中的不一样
