Support pkg-config when mapnik-config is not available

[sebastic]

The CMake build of Mapnik 4.0.0 doesn't provide mapnik-config, it does provide pkg-config support.

This patch is used in the Debian package to use pkg-config unconditionally:

--- a/backend-mapnik/Makefile
+++ b/backend-mapnik/Makefile
@@ -1,8 +1,7 @@
 INSTALLOPTS=-g root -o root
-CFLAGS += -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64
-CXXFLAGS = `mapnik-config --cflags` $(CFLAGS)
+CXXFLAGS += `pkg-config --cflags libmapnik` -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64
 CXXFLAGS += -Wall -Wextra -pedantic -Wredundant-decls -Wdisabled-optimization -Wctor-dtor-privacy -Wnon-virtual-dtor -Woverloaded-virtual -Wsign-promo -Wold-style-cast
-LDFLAGS= `mapnik-config --libs --ldflags --dep-libs`
+LDFLAGS += `pkg-config --libs libmapnik` -lboost_filesystem

 backend-mapnik: renderd.o metatilehandler.o networklistener.o networkmessage.o networkrequest.o networkresponse.o debuggable.o requesthandler.o
        $(CXX) -o $@ $^ $(LDFLAGS)

Some heuristics to determine which to use might be in order.