fix graphics/mapnik regression tests

Hi,
While looking at Omar's diff for SCons update I've found out
that regression tests won't run because they are linked
with libmapnik.so.3.1. The patch below links test with the actual
library.
OK?

Index: patches/patch-src_build_py
===================================================================
RCS file: /cvs/ports/graphics/mapnik/patches/patch-src_build_py,v
retrieving revision 1.7
diff -u -p -u -r1.7 patch-src_build_py
--- patches/patch-src_build_py 10 Jun 2021 19:16:17 -0000 1.7
+++ patches/patch-src_build_py 27 Aug 2021 21:01:45 -0000
@@ -21,7 +21,15 @@ Index: src/build.py

lib_env['LIBS'].append('z')

-@@ -140,7 +137,7 @@ else: # unix, non-macos
+@@ -133,14 +130,14 @@ if env['PLATFORM'] == 'Darwin':
+ else: # unix, non-macos
+ mapnik_libname = env.subst(env['MAPNIK_LIB_NAME'])
+ if env['ENABLE_SONAME']:
+- mapnik_libname = env.subst(env['MAPNIK_LIB_NAME']) + (".%d.%d" %
(int(ABI_VERSION[0]),int(ABI_VERSION[1])))
++ mapnik_libname = env.subst(env['MAPNIK_LIB_NAME']) +
(".${LIBmapnik_VERSION}")
+ if env['PLATFORM'] == 'SunOS':
+ if env['CXX'].startswith('CC'):
+ mapnik_lib_link_flag += ' -R. -h %s' % mapnik_libname
else:
mapnik_lib_link_flag += ' -Wl,-h,%s' % mapnik_libname
else: # Linux and others