build system: relink ksh/shcomp if libraries change (re: 4eaf25c)

When libast.a changed, ksh or shcomp were not getting relinked. To
make that happen, we have to repeat the 'bind -lshell' command,
which acts like 'prev' not just for libshell.a but also for all its
previously built dependencies from other directories.

Similarly, libcmd.a needed an extra 'bind -lsum' to re-extract
libsum.o and re-generated libcmd.a when libsum.a changes.

This commit also removes a few superflous 'prev' commands from some
Mamfiles and updates README-mamake.md with more complete info about
the effect of the 'bind' command.

src/cmd/INIT/README-mamake.md

@@ -379,12 +379,17 @@ causes a MAM variable `mam_lib`*libraryname* to be defined (see **MAM variables*
 The variable will contain either the compiler argument for linking to the library *libraryname*
 (either the `-l`*libraryname* flag, or the full path in case of a static library)
 or, if the `dontcare` attribute is specified, possibly the empty string.
+Any library dependencies are also included (see below).
 This can be used both for AST libraries shipped with the distribution and for system libraries.
-If the library file is found in the distribution,
-its time stamp is checked and the current target is marked as outdated if it is newer.
-
-The effect of `bind` is global, not scoped; it takes effect for all commands
-physically following it, regardless of `make`…`done` nesting level.
+For each corresponding *.a library archive dependency built previously,
+its time stamp is checked and the current target is marked as outdated if it is
+newer, as if a `prev` had been executed for it.
+
+The variable set by `bind` is global, but the marking of the target as
+outdated applies to the current rule only, so it may be necessary to
+repeat a `bind` command when statically linking executables that depend
+on a library, otherwise they may not be relinked when the library changes.
+The `mam_lib`*libraryname* variable will not be regenerated when repeating a `bind`.
 
 There is also a mechanism to communicate library dependency information across Mamfiles and `mamake` invocations.
 If a file named *libraryname*`.req` in the current directory

src/cmd/ksh93/Mamfile

@@ -1195,7 +1195,7 @@ make install virtual
 	shim - }
 
 	make ksh
-		prev libshell.a
+		bind -lshell
 		make pmain.o
 			prev sh/pmain.c
 			exec - ${compile} ${<}
@@ -1204,7 +1204,7 @@ make install virtual
 	done ksh
 
 	make shcomp
-		prev libshell.a
+		bind -lshell
 		make shcomp.o
 			prev sh/shcomp.c
 			exec - ${compile} ${<}
@@ -1216,7 +1216,6 @@ make install virtual
 	note * Pre-install
 	note *
 
-	prev libshell.a
 	loop DIR bin include/ast lib/lib man/man1 man/man3 fun etc
 		make ${INSTALLROOT}/${DIR}
 			exec - mkdir -p ${@}

src/lib/libast/Mamfile

@@ -4931,8 +4931,6 @@ make install virtual
 	note * pre-install libast in $INSTALLROOT
 	note *
 
-	prev libast.a
-
 	note * ensure preinstallation directories exist
 	loop DIR include/ast lib/lib lib/file man/man3
 		make ${INSTALLROOT}/${DIR}

src/lib/libcmd/Mamfile

@@ -463,6 +463,7 @@ make install virtual
 			done
 		done
 		make sumlib.o
+			bind -lsum
 			exec - set -- ${mam_libsum}
 			exec - ${AR} x "$1" sumlib.o
 		done

src/lib/libsum/Mamfile

@@ -143,7 +143,6 @@ make install virtual
 	note * pre-install
 	note *
 
-	prev libsum.a
 	loop DIR ${INSTALLROOT}/lib/lib ${PACKAGE_ast_INCLUDE}
 		make ${DIR}
 			exec - mkdir -p ${@}