build: only use @SET_MAKE@ in the top-level Makefile.in

@SET_MAKE@ (AC_PROG_MAKE_SET) is a legacy autoconf substitution needed
only for very old Unix make implementations that don't automatically
propagate $(MAKE) to recursively invoked sub-makes. Every Makefile.in
in the tree carried its own copy, but that's unnecessary: exporting
MAKE once, from the top-level Makefile.in, is sufficient for it to
reach any sub-make invoked (directly or transitively) from there.

Replace the top-level @SET_MAKE@ with the pair "@SET_MAKE@" followed
by "export MAKE", and drop the now-redundant @SET_MAKE@ line from
doc/Makefile.in, misc/Makefile.in and test/Makefile.in.

This incidentally fixes doc/Makefile.in being usable when fed directly
to nmake (as Mkfiles/msvc.mak's "docs" rule does, since it predates
running ./configure): a bare "@SET_MAKE@" line, left unexpanded, is
not a valid nmake assignment or rule and previously caused a hard
parse error (U1035, "expected ':' or '=' separator").

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Makefile.in

@@ -5,6 +5,7 @@
 # Auto-configuring Makefile for the Netwide Assembler.
 
 @SET_MAKE@
+export MAKE
 
 PACKAGE_NAME    = @PACKAGE_NAME@
 PACKAGE_TARNAME = @PACKAGE_TARNAME@

doc/Makefile.in

@@ -6,8 +6,6 @@
 # UNIX Makefile for NASM documentation
 #
 
-@SET_MAKE@
-
 PACKAGE_NAME    = @PACKAGE_NAME@
 PACKAGE_TARNAME = @PACKAGE_TARNAME@
 

misc/Makefile.in

@@ -6,8 +6,6 @@
 # Makefile for various extra NASM programs and files
 #
 
-@SET_MAKE@
-
 top_srcdir	= @top_srcdir@
 srcdir		= @srcdir@
 top_objdir	= @top_builddir@

test/Makefile.in

@@ -1,5 +1,3 @@
-@SET_MAKE@
-
 .SUFFIXES: .bin .bin16 .bin32 .bin64 .o .o64 .aout .aoutb .obj .obj64 \
 	   .mo32 .mo64 .dbg .i .od .ox .ith .srec .exe .lst \
 	   .asm .pl