Omit duplicate defines in C Headers

[stefanlippuner]

Hey @tgingold-cern

This is a follow-up for #44

Changes:

• Fix C constants generation with same c_name between register and field
  • Same issue as in Fix constants generation with same c_name between register and field #44 but for C headers
  • Add test case for this issue.
• Add more C syntax checks using gcc -fsyntax-only -Wall -Werror (check_c_syntax function).
• Add #include <stdint.h> to C header files. This is required for the uint32_t/... types.

@lorenzschmid FYI

Related to #45

[stefanlippuner]

Based on feedback from @lorenzschmid I've also enabled the same-label test for VHDL/V/SV constants generation.

[tgingold-cern]

Thanks (and sorry for the delay).

[stefanlippuner]

No worries. Thanks for having a look and merging this! :)