In libraries.m4, we check several libraries using AC_CHECK_LIB. However, we misuse the function so it does not do what we think it does.

Inproper use:
AC_CHECK_LIB(jpeg, main, ...)

Proper use:
AC_CHECK_LIB(z, compress, ...)

This is proper since compress is a function in the libz library, but "main" is a function not in libjpeg, but in the autoconf testing program.

In other words, we need to supply a function present in the library we test, or the test will be meaningless.

We currently have improper uses of AC_CHECK_LIB at libjpeg and libgif.