fix pg_config error for darwin os

[msitu]

Issue: #4170

Fixes the following error when trying to run ./configure on mac OSX 10.15.6:

Checking for requested plugins dependencies...
Checking for pg_config... yes
Checking for pg_config... yes
TypeError: a bytes-like object is required, not 'str':
  File "/Users/msitu/src/improc/icin/geotizzler/mapnik/SConstruct", line 1652:
    if conf.parse_config('GDAL_CONFIG',checks='--libs'):
  File "/Users/msitu/src/improc/icin/geotizzler/mapnik/scons/scons-local-3.0.1/SCons/SConf.py", line 659:
    ret = self.test(context, *args, **kw)
  File "/Users/msitu/src/improc/icin/geotizzler/mapnik/SConstruct", line 619:
    if value and '-framework GDAL' in value:
Checking for gdal-config --libs... %   

[codecov]

Codecov Report

Merging #4171 into v3.0.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           v3.0.x    #4171   +/-   ##
=======================================
  Coverage   67.02%   67.02%           
=======================================
  Files         446      446           
  Lines       22324    22324           
=======================================
  Hits        14963    14963           
  Misses       7361     7361           

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