Revert "Remove MPICHLIB_LDFLAGS/LIBS"

This reverts commit
4d93cefa8f. pmodels/mpich#6904 has a valid
use-case that we should support. An additional commit will modify the
compile scripts so these flags will not leak out of the build.

README.vin

@@ -305,10 +305,10 @@ MPICH library being compiled/linked with these flags and the flags
 internally being used in mpicc and friends.
 
 (b) MPICHLIB_CFLAGS, MPICHLIB_CPPFLAGS, MPICHLIB_CXXFLAGS,
-MPICHLIB_FFLAGS, and MPICHLIB_FCFLAGS (abbreviated as
-MPICHLIB_xFLAGS): Setting these flags would result in the MPICH
-library being compiled with these flags.  However, these flags will
-*not* be used by mpicc and friends.
+MPICHLIB_FFLAGS, MPICHLIB_FCFLAGS, MPICHLIB_LDFLAGS and
+MPICHLIB_LIBS (abbreviated as MPICHLIB_xFLAGS): Setting these flags
+would result in the MPICH library being compiled/linked with these
+flags. However, these flags will *not* be used by mpicc and friends.
 
 (c) MPICH_MPICC_CFLAGS, MPICH_MPICC_CPPFLAGS, MPICH_MPICC_LDFLAGS,
 MPICH_MPICC_LIBS, and so on for MPICXX, MPIF77 and MPIFORT

configure.ac

@@ -302,11 +302,17 @@ AC_ARG_VAR(MPICHLIB_FFLAGS,
 	[extra FFLAGS used in building MPICH libraries])
 AC_ARG_VAR(MPICHLIB_FCFLAGS,
 	[extra FCFLAGS used in building MPICH libraries])
+AC_ARG_VAR(MPICHLIB_LDFLAGS,
+	[extra LDFLAGS used in building MPICH libraries])
+AC_ARG_VAR(MPICHLIB_LIBS,
+	[extra LIBS used in building MPICH libraries])
 CFLAGS="$CFLAGS $MPICHLIB_CFLAGS"
 CPPFLAGS="$CPPFLAGS $MPICHLIB_CPPFLAGS"
 CXXFLAGS="$CXXFLAGS $MPICHLIB_CXXFLAGS"
 FFLAGS="$FFLAGS $MPICHLIB_FFLAGS"
 FCFLAGS="$FCFLAGS $MPICHLIB_FCFLAGS"
+LDFLAGS="$LDFLAGS $MPICHLIB_LDFLAGS"
+LIBS="$LIBS $MPICHLIB_LIBS"
 
 # Save a copy of precious flags as USER_* before any of these flags
 # are being modified by configure tests.