During upgrade to sage 5.7 ppl is using gmpxx headers from the system

[fbissey]

This is a problem reported by user Pong on sage-devel ​https://groups.google.com/forum/?hl=en&fromgroups=#!topic/sage-devel/iVudo_7dSFU From hi failure log it appears that ppl tries to use the system gmpxx headers. which in this case is from gmp 5.1.0 which is imcompatible with this version ppl see ​https://bugs.gentoo.org/show_bug.cgi?id=447928 for example:

make[5]: Entering directory `/home/pong/sage-current/spkg/build/ppl-0.11.2.p1/src/src'
/bin/sh ../libtool  --tag=CXX   --mode=compile g++ -DHAVE_CONFIG_H -I. -I..  -I.. -I../src    -g -O2 -frounding-math  -I/home/pong/sage-current/local/include -W -Wall -MT Box.lo -MD -MP -MF .deps/Box.Tpo -c -o Box.lo Box.cc
libtool: compile:  g++ -DHAVE_CONFIG_H -I. -I.. -I.. -I../src -g -O2 -frounding-math -I/home/pong/sage-current/local/include -W -Wall -MT Box.lo -MD -MP -MF .deps/Box.Tpo -c Box.cc  -fPIC -DPIC -o .libs/Box.o
In file included from checked.defs.hh:29:0,
                 from Checked_Number.defs.hh:28,
                 from Coefficient.types.hh:16,
                 from Coefficient.defs.hh:27,
                 from Box.defs.hh:29,
                 from Box.cc:25:
mp_std_bits.defs.hh:48:7: error: redefinition of 'class std::numeric_limits<__gmp_expr<__mpz_struct [1], __mpz_struct [1]> >'
/usr/include/gmpxx.h:3269:21: error: previous definition of 'class std::numeric_limits<__gmp_expr<__mpz_struct [1], __mpz_struct [1]> >'
mp_std_bits.defs.hh:109:7: error: redefinition of 'class std::numeric_limits<__gmp_expr<__mpq_struct [1], __mpq_struct [1]> >'
/usr/include/gmpxx.h:3306:21: error: previous definition of 'class std::numeric_limits<__gmp_expr<__mpq_struct [1], __mpq_struct [1]> >'