Some crypto libraries (like CyaSSL, MatrixSSL and CommonCrypto) either
don't have or expose this functionality at all, expose only part of it,
or (like OpenSSL) have heavyweight API for it. Also, for the task as
easy as BASE64 encoding and decoding it's much better to use small and
simple specialized library.
Add previously missing -Wall to warning flags in CMake. Remove -Wformat
and -Wvariadic-macros (enabled by default; latter is not meaningful in
C99 mode we use), -Wdeclaration-after-statement (again, not needed as
we use C99). Move -Wmissing-declarations to C-only flags (GCC man says
so).
Add copyrights year to crypto-utils-fallback.c.
