Blosc changed the signature of blosc_compcode_to_compname in 1.15 so
that the second parameter is now const char** not char **. This causes
compile failures when using openvdb with earlier versions of blosc.

The fix, which is backported from openvdb-7, is to check the blosc
version and cast the char** to const char** for modern versions of
blosc.

The bug can be produced by emerging blosc 1.15+, and then openvdb.
Without this patch, the compiler will fail with
error: invalid conversion from ‘char**’ to ‘const char**’

To test that the patch has been properly implemented, emerge modern
blosc and then openvdb with this patch, and the compile will succeed.

Fixes bug https://bugs.gentoo.org/734102
Upstream commit https://github.com/AcademySoftwareFoundation/openvdb/commit/d2e8bd87a63d1e9f66a558ecbb6e6cbd54f7de13

diff -Naur a/openvdb/unittest/TestFile.cc b/openvdb/unittest/TestFile.cc
--- a/openvdb/unittest/TestFile.cc	2020-08-19 02:11:48.945711312 +1000
+++ b/openvdb/unittest/TestFile.cc	2020-08-19 02:13:31.106074899 +1000
@@ -2666,7 +2666,12 @@
 
     for (int compcode = 0; compcode <= BLOSC_ZLIB; ++compcode) {
         char* compname = nullptr;
-        if (0 > blosc_compcode_to_compname(compcode, &compname)) continue;
+#if BLOSC_VERSION_MAJOR > 1 || (BLOSC_VERSION_MAJOR == 1 && BLOSC_VERSION_MINOR >= 15)
+        if (0 > blosc_compcode_to_compname(compcode, const_cast<const char**>(&compname)))
+#else
+        if (0 > blosc_compcode_to_compname(compcode, &compname))
+#endif
+        continue;
         /// @todo This changes the compressor setting globally.
         if (blosc_set_compressor(compname) < 0) continue;