configure fails reporting unable to find library libpng.so.2 in module conftest.

From debug.log:

gcc -o conftest -g -O2  -D_POSIX_PTHREAD_SEMANTICS  -R/var/local/mysql/lib/mysql
 -L/var/local/mysql/lib/mysql conftest.c -lmysqlclient -lpng -lz -lgd -lresolv -
lresolv -lresolv -lm -ldl -lcrypt -lnsl -lsocket  -lsocket -lgcc 1>&5
ld.so.1: ./conftest: fatal: libpng.so.2: open failed: No such file or directory
~

The PNG library is installed in /usr/local/lib and LDFLAGS is set to /usr/local/lib to pick up the path.

after configure fails, config.log shows multiple errors referencing pthreads.  This is only the first one, others are similar:

configure:2442: gcc -o conftest -g -O2  -D_POSIX_PTHREAD_SEMANTICS  conftest.c
1>&5
Undefined                       first referenced
 symbol                             in file
pthread_create                      /var/tmp/ccPCuAOp.o
ld: fatal: Symbol referencing errors. No output written to conftest
collect2: ld returned 1 exit status
configure: failed program was:
#line 2424 "configure"
#include "confdefs.h"
(more omitted)

Do PHP4 require an external thread library such as MIT pthreads?  

PHP3 builds with no problems and I've been running it for months.

Thanks in advance.