Fix failures when used with an expression-template enabled number type such as the gmpxx clases.

Add additional concept check for integer code using gmp classes.
Fixes #4139.

[SVN r61837]

libs/math

@@ -1 +1 @@
-Subproject commit c7e0965ce9f70926884eec6e211118388bdefbcf
+Subproject commit 7ab7d21eaec11cbef821f9b7385a8b39565697e4