(I need to build gcc to build beta version of gpc)
I have tried plain DOS and bash shell(bsh1147b) (make 3.761 and make 3.75): 
same result.
>Configure go32
>make
complains because of the backslash \ in line 1273 in the makefile of the 
%DJGPP%\gnu\gcc-2721 directory:

	bi-arity < $(srcdir)ytecode.def >t-bc-arity.h

(and similar issues in lines 1279 and 1285)
In the makefile.in in the same directory from which this makefile is built 
there is correctly a forward slash /
	bi-arity < $(srcdir)/bytecode.def >t-bc-arity.h

This seems to be produced by the application of the top.sed in the 
config\msdos subdirectory which contains the line

s/<\ *\$(srcdir)\//< $(srcdir)\/g

which effect is indeed to substitute:  < $(srcdir)/  by:  < $(srcdir)\

removing this line in top.sed eliminates the complain: but this may have other
effects elsewhere. And I really do not understand the purpose of this line if it
is not to replace / by \  It seems to be purely cosmetic by removing extra
spaces between < and $. 
Also the makefile contained in the source distribution has forward slashes
but it is overridden by running configure go32. And I must run configure go32
to buils the gpc compiler.
Any idea ?

There seems to have been two errors in transmission of the preceeding
report, which make it hard to understand:
The first quoted line in the makefile is

	bi-arity < $(srcdir)ytecode.def >t-bc-arity.h

the quoted line in top.sed is

s/<\ *\$(srcdir)\//< $(srcdir)\/g

These quotes are made with cut and paste from my editor 
and I have checked that there are no hidden tabs or control characters
in the files to explain what is going on.

Well... this is not a random error. The same has occurred.
In fact I have found that "escape sequences garble bug reports system"
is already signalled as bug #00049.
If you want to understand, you have to read directly the original files
to see where are the back slashes (really needed to see the 
top.sed line and its backslash backslash construct)