The following excellent articles in the past two days taught me a lot about how the loader does relocation for ELF executables and how thread local storage is handled:

• Load-time relocation of shared libraries
• Position Independent Code (PIC) in shared libraries
• Position Independent Code (PIC) in shared libraries on x64
• A Deep dive into (implicit) Thread Local Storage
• All about thread-local storage

GCC has its version written in the .comment section of the executable binaries as shown by commands:

objdump -s --section .comment /path/to/binary
# or pretty print
readelf -p .comment /path/to/binary

Two useful tips from TACC tips regarding compiler macro expanding.

Tip 120:
Want to know what MACROS are predefined by the GCC and Intel compilers? Do

gcc -dM -E - < /dev/null > gcc_defined_macros.txt
icc -dM -E - < /dev/null > icc_defined_macros.txt

The flag -E in the tip is telling the compiler to expand macros.

Tip 69:
Having trouble figuring out how a macro is being expanded? Try pasting the line into a terminal and adding a -E. For example try changing mpicc -DFOO -DBAR -c foo.c into mpicc -DFOO -DBAR -E foo.c > foo.i and look at the foo.i file.