Merge from trunk:

* Fixed incorrect usage of Boost.Math when it does not support long double (fixes #8162)
* Fixed ambiguity of boost::lexical_cast functions (fixes #7421)

[SVN r83542]

libs/conversion

@@ -1 +1 @@
-Subproject commit 335c868e9b276cde41b4a94b61699294d01e2194
+Subproject commit a4bd056b8ef186f2ec8a27f53f41cf6e151ccd53