I'm using shared libgd.so.2.0.0 which supports jpeg, xpm, png and libz. Unfortunately php configure does NOT include
libpng and libz in its test, so it gets wrong results.
E.g.:

configure:15759: checking whether to include GD support
configure:16388: checking for gdImageString16 in -lgd
configure:16407: gcc -o conftest -g -O2  -DLINUX=2 -DMOD_SSL=206105 -DEAPI -DUSE
_EXPAT -DSHARED_CORE  -Wl,-rpath,/opt/ssl/lib -L/opt/ssl/lib -Wl,-rpath,/usr/X11
R6/lib -L/usr/X11R6/lib -Wl,-rpath,/usr/local/lib -L/usr/local/lib -L/usr/local/
lib conftest.c -lgd  -lgd -lX11 -lXpm -ljpeg -lcrypto -lssl -lresolv -lm -ldl -l
crypt -lnsl  -lresolv -L/usr/lib -ljpeg -L/usr/X11R6/lib -lXpm -L/usr/X11R6/lib 
-lX11 1>&5
/usr/local/lib/libgd.so: undefined reference to `uncompress'
/usr/local/lib/libgd.so: undefined reference to `png_get_rowbytes'
/usr/local/lib/libgd.so: undefined reference to `png_set_strip_16'
/usr/local/lib/libgd.so: undefined reference to `png_create_read_struct'
...
collect2: ld returned 1 exit status
configure: failed program was:
...