CONFIG: improve support for compiler/link options (#1830)

- introduce WM_COMPILE_CONTROL variable to convey control information
  into the build rules.

  The convention (as per spack):
      - '+' to select a feature
      - '~' to deselect a feature

  Eg, to select the gold linker, and disable openmp
  (spaces are not required):

      WM_COMPILE_CONTROL="+gold ~openmp"

CONFIG: accept FOAM_EXTRA_LDFLAGS for AMD, gold, Mingw linkers

CONFIG: generalize PROJECT_LIBS (-ldl used almost universally)

wmake/rules/solaris64Gcc/general

@@ -1,10 +1,14 @@
 CPP        = /lib/cpp
 LD         = ld -64
 
-PROJECT_LIBS = -l$(WM_PROJECT) -lnsl -lsocket -L$(FOAM_LIBBIN)/dummy -lPstream
+PROJECT_LIBS = -l$(WM_PROJECT) \
+    -lnsl -lsocket \
+    -L$(FOAM_LIBBIN)/dummy -lPstream
 
 include $(GENERAL_RULES)/standard
 
 include $(DEFAULT_RULES)/X
 include $(DEFAULT_RULES)/c
 include $(DEFAULT_RULES)/c++
+
+#------------------------------------------------------------------------------