ldconfig.

Creating /etc/ld.so.conf

Some softwares need to set the shared library in order to run. Here is part of the INSTALL file from qwt-5.0.2.

To install examples:
cd examples
qmake
make

If you like to run the examples, don't forget to install the
Qwt library/plugins or set the LD_LIBRARY_PATH to the lib directory
of your local build. ( On some systems LIBPATH is used instead, on MacOSX
it is called DYLD_LIBRARY_PATH. )

The above can be done by:
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/qwt-5.0.2/lib/
Or by the following way--modify ld.so.conf

• Create a new file /etc/ld.so.conf containing:

# Begin /etc/ld.so.conf

/lib
/usr/lib
/usr/X11R6/lib
/usr/local/qwt-5.0.3/lib

# End /etc/ld.so.conf

• Update the dynamic loader cache by running:

ldconfig

Also check "man ldconfig"
There is a nice article about "shared library"