PKG_CONFIG_PATH is a environment variable that specifies additional paths in which pkg-config will search for its .pc files.
This variable is used to augment pkg-config's default search path. On a typical Unix system, it will search in the directories /usr/lib/pkgconfig and /usr/share/pkgconfig. This will usually cover system installed modules. However, some local modules may be installed in a different prefix such as /usr/local. In that case, it's necessary to prepend the search path so that pkg-config can locate the .pc files.
The pkg-config program is used to retrieve information about installed libraries in the system. The primary use of pkg-config is to provide the necessary details for compiling and linking a program to a library. This metadata is stored in pkg-config files. These files have the suffix .pc and reside in specific locations known to the pkg-config tool.
To check the PKG_CONFIG_PATH value use this command:
echo $PKG_CONFIG_PATH
To set the PKG_CONFIG_PATH value use:
export PKG_CONFIG_PATH=/usr/lib/pkgconfig
or
export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig