[Unix] 如果pkg-config找不到某库的路径

pkg-config的原理是：每个由pkg-config管理的库都会提供一个后缀为.pc的文件，pkg-config不过是把这个文件读出，分析并打印在屏幕上罢了。

比如opencv的pc文件（opencv.pc）:

# Package Information for pkg-config
 
prefix=/usr/local
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir_old=${prefix}/include/opencv
includedir_new=${prefix}/include
 
Name: OpenCV
Description: Open Source Computer Vision Library
Version: 2.2.0
Libs: -L${libdir} -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_ml -lopencv_video -lopencv_features2d -lopencv_calib3d -lopencv_objdetect -lopencv_contrib -lopencv_legacy -lopencv_flann
Cflags: -I${includedir_old} -I${includedir_new}

如果pkg-config找不到某个库的路径，需要做的就是找到该库的pc文件，然后拷到pkg-config指定的路径下就行了。pc文件一般在编译时会生成，pkg-config指定的路径，在osx下如果是用macports安装的pkg-config，则会在/opt/local/lib/pkgconfig。