Portability page updates, FreeBSD clang versions

[svaarala]

• Update FreeBSD clang workaround information
• Link to probable root cause, see duk cores at start with segfault on FreeBSD 11.1-RELEASE duktape#1752 (comment)
• -fno-strict-aliasing workaround
• Duktape 1.x -ism fixes

[michael-o]

Looks good to me, but for one this. It is not clear wether -UDUK_USE_PACKED_TVAL must be passed to make or to configure.py because:

cc -o duk  -UDUK_USE_PACKED_TVAL -Os -pedantic -std=c99 -Wall -fstrict-aliasing -fomit-frame-pointer -I./src-freebsd -DDUK_CMDLINE_PRINTALERT_SUPPORT -I./extras/print-alert -DDUK_CMDLINE_CONSOLE_SUPPORT -I./extras/console -DDUK_CMDLINE_LOGGING_SUPPORT -I./extras/logging -DDUK_CMDLINE_MODULE_SUPPORT -I./extras/module-duktape src-freebsd/duktape.c examples/cmdline/duk_cmdline.c extras/print-alert/duk_print_alert.c extras/console/duk_console.c extras/logging/duk_logging.c extras/module-duktape/duk_module_duktape.c -lm
src-freebsd/duktape.c:11083:24: warning: expression result unused [-Wunused-value]
        DUK_USE_FATAL_HANDLER(udata, msg);
                              ^~~~~
src-freebsd/duktape.c:11083:31: warning: expression result unused [-Wunused-value]
        DUK_USE_FATAL_HANDLER(udata, msg);
                                     ^~~
2 warnings generated.
[mosipov@mika-ion ~/Projekte/duktape-2.1.1]$ ./duk
*** FATAL ERROR: self test(s) failed
Causing intentional segfault...
Segmentation fault (Speicherabzug geschrieben)
[mosipov@mika-ion ~/Projekte/duktape-2.1.1]$

[svaarala]

Ah, the text was not yet finished but just drafting a bit.

As of Duktape 2.x config options are always handled via configure.py, never by defining them on the command line (see http://duktape.org/guide.html#compiling). But I can clarify that on this page, too. The whole page is still pretty much in Duktape 1.x state so I'll reword the text in a few other places too.