configure.in and libgd

[Allin C. <cot...@wf...>]

There's something wrong with the handling of the linker line for
libgd in gnuplot's configure.in.  Consider a system that has a
correct gdlib.config: gnuplot does

libgd_LIBS=`gdlib-config --libs`

That's fine, but it should be the end of the story. However,
further down in configure.in, the script unconditionally adds
"-ljpeg" (when checking for JPEG support) and "-lpng" (when
checking for PNG support).  The problem is that if libgd is linked
against libpng12.so, and if there also exists on the system the
older (or compatibility) library libpng.so, then gnuplot will end
up linked against both libpng12 and libpng.

E.g., gdlib-config --libs gives:

-lXpm -lX11 -lfontconfig -lfreetype -lpng12 -lz -lm

and in src/Makefile we get:

TERMLIBS =  <...> -lz -lgd -lXpm -lX11 \
-lfontconfig -lfreetype -lpng12 -lz -lm -lfreetype -lpng

I'm not sure exactly what's the best fix here, but it seems that
adding "-lpng" should somehow be made conditional on
the libgd_LIBS variable not already including a directive to link
to (some variant of) libpng.

-- 
Allin Cottrell
Department of Economics
Wake Forest University