libpng: zlib path (#5581)
libpng: zlib path (#5581)

explicitly set the zlib path for libpng configure.

fixes:
```
     [ ... ]
     92    checking for memset... yes
     93    checking for pow... no
     94    checking for pow in -lm... yes
     95    checking for clock_gettime... yes
     96    checking for zlibVersion in -lz... no
     97    checking for z_zlibVersion in -lz... no
  >> 98    configure: error: zlib not installed
```
krafczyk/spack,EmreAtes/spack,tmerrick1/spack,iulian787/spack,matthiasdiener/spack,matthiasdiener/spack,iulian787/spack,EmreAtes/spack,EmreAtes/spack,LLNL/spack,krafczyk/spack,LLNL/spack,tmerrick1/spack,skosukhin/spack,lgarren/spack,mfherbst/spack,matthiasdiener/spack,lgarren/spack,krafczyk/spack,iulian787/spack,matthiasdiener/spack,skosukhin/spack,skosukhin/spack,EmreAtes/spack,lgarren/spack,mfherbst/spack,mfherbst/spack,skosukhin/spack,LLNL/spack,tmerrick1/spack,mfherbst/spack,iulian787/spack,iulian787/spack,LLNL/spack,matthiasdiener/spack,LLNL/spack,tmerrick1/spack,krafczyk/spack,lgarren/spack,EmreAtes/spack,lgarren/spack,tmerrick1/spack,mfherbst/spack,krafczyk/spack,skosukhin/spack