clang6 fix: math/libqalculate

[Lawrence Teo]

This lets math/libqalculate build with clang6.

The error is:

/usr/ports/pobj/libqalculate-0.9.7/libqalculate-0.9.7/libqalculate/MathStructure.cc:7166:48:
 error: cannot initialize a parameter of type 'MathStructure *' with an rvalue 
of type 'bool'
        MathStructure::gcd(cont_c, cont_d, gamma, eo, false);
                                                      ^~~~~
The function prototype of MathStructure::gcd in MathStructure.h is:

static bool gcd(const MathStructure &m1, const MathStructure &m2,
    MathStructure &mresult, const EvaluationOptions &eo,
    MathStructure *ca = NULL, MathStructure *cb = NULL,
    bool check_args = true);

Their latest Github code and libqalculate-0.9.8 indicate that it is
meant to be:

MathStructure::gcd(cont_c, cont_d, gamma, eo, NULL, NULL, false);

References:
* 
https://github.com/Qalculate/libqalculate/blob/master/libqalculate/MathStructure.cc#L9883
* libqalculate-0.9.8/libqalculate/MathStructure.cc in
  
https://github.com/Qalculate/libqalculate/files/316260/libqalculate-0.9.8.tar.gz

ok?


Index: patches/patch-libqalculate_MathStructure_cc
===================================================================
RCS file: patches/patch-libqalculate_MathStructure_cc
diff -N patches/patch-libqalculate_MathStructure_cc
--- /dev/null   1 Jan 1970 00:00:00 -0000
+++ patches/patch-libqalculate_MathStructure_cc 15 Apr 2018 19:02:04 -0000
@@ -0,0 +1,14 @@
+$OpenBSD$
+
+Index: libqalculate/MathStructure.cc
+--- libqalculate/MathStructure.cc.orig
++++ libqalculate/MathStructure.cc
+@@ -7163,7 +7163,7 @@ bool sr_gcd(const MathStructure &m1, const MathStructu
+       c.polynomialContent(xvar, cont_c, eo);
+       d.polynomialContent(xvar, cont_d, eo);
+       MathStructure gamma;
+-      MathStructure::gcd(cont_c, cont_d, gamma, eo, false);
++      MathStructure::gcd(cont_c, cont_d, gamma, eo, NULL, NULL, false);
+       if(ddeg.isZero()) {
+               mgcd = gamma;
+               return true;