C – Questions About Linking Libraries in C

C++ templates are a sort of dumbed down Lisp (or even more, Scheme) macros. It is a Turing-complete language which evaluates in compilation time, but it is severely limited as there is no access from that language to the underlying C++ environment. So, yes, C++ templates can be seen as some form of preprocessing, with a very limited interaction with the code being generated.

Short answer: Any use of GCC for compiling code is, always was and always will be safe.

1. __builtin_expect()&co. are mostly not functions but syntax extensions. More similar to + operator or for keyword than library function. For example __builtin_expect() generates no instructions at all, it just tells the compiler to order the branches so that the particular case is more efficient than the other (cpu predicts the jump in particular way, fewer jumps on that path, etc.). Even for those "builtins" that do generate code, it's like any other code generated by primitive language construct. It constitutes normal use of the compiler and normal use is never covered by GPL.
2. However some language constructs, whether defined by C/C++ standard or GCC extensions including any __builtin* and __sync* and such may generate calls to the gcc library (i.e. libgcc.a). This library is covered by GPL, but a special exception is provided that allows linking it with any code and distributing the resulting binary under any license whatsoever.
3. The same exception applies to GNU stdlibc++ (libstdc++.a/libstdc++.so), so you are safe with standard C++ library as well.
4. The standard C library (which comes with operating system, not gcc), GNU libc (in case of Linux), is however covered by LGPL, which only allows dynamic linking of code under different license. Since standard C library contains the system call interface that is often updated to take advantage of improvements in newer kernels it is not recommended to always link this one dynamically anyway.