build: pass pkg_config_path to check_cfg() when looking for libndn-cxx

Instead of modifying os.environ.

This commit also deletes two unused waf tools and fixes the value of
includedir in the generated .pc file.

Refs: #5042
Change-Id: I6be742dea1367c90f8b242d4fc6e51bc498fbf06
diff --git a/.waf-tools/default-compiler-flags.py b/.waf-tools/default-compiler-flags.py
index b69ca79..9e045c3 100644
--- a/.waf-tools/default-compiler-flags.py
+++ b/.waf-tools/default-compiler-flags.py
@@ -197,6 +197,8 @@
         version = self.getCompilerVersion(conf)
         if version < (3, 9, 0) or (Utils.unversioned_sys_platform() == 'darwin' and version < (8, 1, 0)):
             flags['CXXFLAGS'] += ['-Wno-unknown-pragmas']
+        if version < (6, 0, 0):
+            flags['CXXFLAGS'] += ['-Wno-missing-braces'] # Bug #4721
         return flags
 
     def getOptimizedFlags(self, conf):
@@ -209,4 +211,6 @@
         version = self.getCompilerVersion(conf)
         if version < (3, 9, 0) or (Utils.unversioned_sys_platform() == 'darwin' and version < (8, 1, 0)):
             flags['CXXFLAGS'] += ['-Wno-unknown-pragmas']
+        if version < (6, 0, 0):
+            flags['CXXFLAGS'] += ['-Wno-missing-braces'] # Bug #4721
         return flags