Fix Solaris issues

[jdemeyer]

This fixes two issues when compiling on Solaris with GCC-5.4.0.

The first is an issue with pow():

nr/dpe.h: In function 'void dpe_pow_si(dpe_struct*, const dpe_struct*, unsigned int)':
nr/dpe.h:718:35: error: call of overloaded 'pow(const double&, const unsigned int&)' is ambiguous
   DPE_MANT(x) = pow(DPE_MANT(y), k);

The second is GCC bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10200
The existence of a templated norm function causes GCC to misparse if (p->norm < v->norm)

[codecov]

Codecov Report

Merging #348 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #348   +/-   ##
=======================================
  Coverage   73.87%   73.87%           
=======================================
  Files          55       55           
  Lines        4891     4891           
=======================================
  Hits         3613     3613           
  Misses       1278     1278

Impacted Files	Coverage Δ
fplll/sieve/sieve_gauss_3sieve.cpp	91.26% <100%> (ø)	⬆️
fplll/sieve/sieve_gauss_4sieve.cpp	87.5% <100%> (ø)	⬆️
fplll/nr/dpe.h	81.87% <100%> (ø)	⬆️
fplll/sieve/sieve_gauss_2sieve.cpp	96.07% <100%> (ø)	⬆️

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[malb]

Thank you!