After reading this file, you should read README.orig.

To build GNU libg++ 2.5.3, you'll need to do a few things
first.  

First, make sure that gcc is installed, and that you have
configured it for C++.  The Skunkworks gcc is already built
that way.

Run "./configure --prefix=/usr/local --target=i486-unknown-sco3.2v4.2"

The Makefile will be generated with some macros which cause
some problems.  Edit the Makefile.  You'll see something like

   CXX_FOR_TARGET = ` \
     if [ -f $${rootme}/gcc/Makefile ] ; then \
       echo $${rootme}/gcc/xgcc -B$${rootme}/gcc/; \
     else \
       if [ "$(host_canonical)" = "$(target_canonical)" ] ; then \
         echo $(CXX); \
       else \
         t='$(program_transform_name)'; echo gcc | sed -e '' $$t; \
       fi; \
     fi`

You want to change this to

    CXX_FOR_TARGET=$(CXX)

The full list of macros like this that you should change is:
	CXX_FOR_TARGET = $(CXX)
	CC_FOR_TARGET = $(CC)
	CC_FOR_BUILD = $(CC)
	CXX_FOR_BUILD = $(CXX)
	AS_FOR_TARGET = $(AS)
	AR_FOR_TARGET = $(AR)
	RANLIB_FOR_TARGET = $(RANLIB)
	NM_FOR_TARGET = $(NM)

Next, edit libg++/src/builtin.h and comment out lines which
mention the "abs" and "sqr" functions, as these conflict with
stuff in <math.h>.

Now, run

	make

and then

	make install