build: avoid name transformations on libstdbuf

* src/local.mk (transform): Skip the transformation for libstdbuf
since that should not be subject to name clashes, and we need
to reference the name directly in LD_PRELOAD etc.
* configure.ac: Add a comment on the coupling of pkglibexec_PROGRAMS
to $(transform).
Issue reported at https://trac.macports.org/ticket/44922
Improved by Nick Bowler

configure.ac

@@ -488,6 +488,8 @@ m4_include([m4/cu-progs.m4])
 
 # Now that we know which programs will actually be built, determine
 # which optional helper progs should be compiled.
+# Note it adding to pkglibexec_PROGRAMS, $(transform) in src/local.mk
+# may need to be updated accordingly.
 case " $optional_bin_progs " in
   *' stdbuf '*) pkglibexec_PROGRAMS='src/libstdbuf.so';;
   *) pkglibexec_PROGRAMS='';;

src/local.mk

@@ -336,11 +336,18 @@ copy_sources = \
   src/extent-scan.c \
   src/extent-scan.h
 
+# Don't apply prefix transformations to libstdbuf shared lib
+# as that's not generally needed, and we need to reference the
+# name directly in LD_PRELOAD etc.  In general it's surprising
+# that $(transform) is applied to libexec at all given that is
+# for internal package naming, not privy to $(transform).
+
 # Use 'ginstall' in the definition of PROGRAMS and in dependencies to avoid
 # confusion with the 'install' target.  The install rule transforms 'ginstall'
 # to install before applying any user-specified name transformations.
 
-transform = s/ginstall/install/; $(program_transform_name)
+transform = /libstdbuf/q;s/ginstall/install/;$(program_transform_name)
+
 src_ginstall_SOURCES = src/install.c src/prog-fprintf.c $(copy_sources) \
 		       $(selinux_sources)