try to link symlink and use a define on error

fixes compiling with Minimalist GNU win32 (cross) compiler 4.2.1, which
defines symlink, but doesn't implement it.
workaround for missing #warning on windows
build silently by default
check for alloca
check for strcasecmp
Include the module name in the extracted ASN.1 file.
Declare the permitted alphabet tables and code2value tables const.
Declare the type descriptor and input buffer args of some functions as const.
Declare most internal, constant tables as const, particularly the
"specifics" structures and the tables they point to.
Declare the tags[] tables in skeletons as const.
Constify the PER support structure.
Constify a few other private tables in the skeleton code.
Avoid an unnecessary table lookup and its accompanying table (final_chunk_type[]).
It's actually more efficient *not* to create a mutable static variable to hold this constant string's address.
improved language
clarification
travis CI integration
version 0.9.26
ignore temporary files
portability fixes
bump up asn1c version
bump up asn1c version