Variable exports are performed over multiple directory levels from parent- to
subdirectory-makefiles.

To avoid specifying them at many locations, these exports are defined in files named
export2sub.  Currently these export2sub exist:

        ../SOURCE_TOOLS/export2sub
        ../UNIT_TESTER/export2sub
        ../AISC_COM/AISC/export2sub


Short description of most important variables passed from parent makefiles:

      A_CC                C-compiler
      A_CXX               C++-compiler

      cflags              C-compiler flags
      cxxflags            additional C++-compiler flags
                          (should be used as '$(cflags) $(cxxflags)'!)
      shared_cflags       additional compiler flags used when compiling objects for a shared library

      CC_INCLUDES         include directories for C-compiler
      CXX_INCLUDES        include directories for C++-compiler

      POST_COMPILE        global filter applied to compiler output (i.e. to errors and warnings)

      LINK_EXECUTABLE     linker command for executables
      LINK_STATIC_LIB     linker command for static libs
      LINK_SHARED_LIB     linker command for shared libs (depends on LINK_STATIC)

      LINK_STATIC         =0 -> use dynamic libs, =1 -> use only static libs
      SHARED_LIB_SUFFIX   suffix for shared libs (contains .a if LINK_STATIC=1)

  Several dynamic library definitions are documented in ../Makefile@DYNLIBDEFS

Note:
        Several variables are still passed directly via the call of the submakefile, e.g.
                make -C subdir "VAR=content" target
        If passing them is common at many places, they should better be passed via export2sub