build: actually set LDFLAGS and LIBS in makefiles

Both variables are used inside on src/prog.mk and src/so.mk, but they
are not currently defined in any makefile, so their values cannot be
substituted by ./configure.

This means that the variables can be set when running make (such as with
`make LDFLAGS=-Lfoo`), but changing them in configure.ac has no effect.
The same applies when trying to set them when running ./configure (such
as with `./configure LDFLAGS=-Lfoo`).

config.mk.in

@@ -53,6 +53,8 @@ MANFLAGS = $(HAVE_LTS) $(HAVE_OUTPUT) $(HAVE_X11) $(HAVE_PRIVATE_HOME) $(HAVE_AP
 
 CC=@CC@
 CFLAGS=@CFLAGS@
+LDFLAGS=@LDFLAGS@
+LIBS=@LIBS@
 
 ifdef NO_EXTRA_CFLAGS
 else

configure

@@ -5260,8 +5260,10 @@ cat <<EOF
 Compile options:
    CC: $CC
    CFLAGS: $CFLAGS
+   LDFLAGS: $LDFLAGS
    EXTRA_CFLAGS: $EXTRA_CFLAGS
    EXTRA_LDFLAGS: $EXTRA_LDFLAGS
+   LIBS: $LIBS
    fatal warnings: $HAVE_FATAL_WARNINGS
    gcov instrumentation: $HAVE_GCOV
    install as a SUID executable: $HAVE_SUID

configure.ac

@@ -293,8 +293,10 @@ cat <<EOF
 Compile options:
    CC: $CC
    CFLAGS: $CFLAGS
+   LDFLAGS: $LDFLAGS
    EXTRA_CFLAGS: $EXTRA_CFLAGS
    EXTRA_LDFLAGS: $EXTRA_LDFLAGS
+   LIBS: $LIBS
    fatal warnings: $HAVE_FATAL_WARNINGS
    gcov instrumentation: $HAVE_GCOV
    install as a SUID executable: $HAVE_SUID