ENH: relocate WM_PROJECT_SITE default (issue #1050)

- was PREFIX/site, now PROJECT/site

  This avoids several issues when installing OpenFOAM in clusters
  without an intermediate OpenFOAM-specific installation prefix.

  The 'site' directory may have a reserved meaning in these situations
  and it is undesirable to 'leak' upwards into the parent directory to
  look for configuration files.

  Placing the default within the project directory avoids this.
  Alternative locations can be given via the WM_PROJECT_SITE variable.

etc/config.sh/settings

@@ -148,7 +148,7 @@ if [ -n "$WM_PROJECT_SITE" ]
 then
     siteDir=$WM_PROJECT_SITE
 else
-    siteDir=$WM_PROJECT_INST_DIR/site
+    siteDir=$WM_PROJECT_DIR/site
 fi
 
 # Shared site (group) executables/libraries
@@ -173,7 +173,6 @@ if [ -d "$siteDir/$WM_PROJECT_VERSION/bin" ]    # Version-specific
 then
     _foamAddPath "$siteDir/$WM_PROJECT_VERSION/bin"
 fi
-unset siteDir
 
 _foamAddPath $FOAM_USER_APPBIN:$FOAM_SITE_APPBIN:$FOAM_APPBIN
 _foamAddLib  $FOAM_LIBBIN/dummy     # Dummy versions of external libraries last
@@ -321,7 +320,7 @@ esac
 
 # Cleanup
 # ~~~~~~~
-unset archDir
+unset archDir siteDir
 unset gcc_version gccDir
 unset gmp_version gmpDir  mpfr_version mpfrDir  mpc_version mpcDir
 unset clang_version clangDir