I can't find a reference anywhere to a header containing the __builtin_stdarg_start but attempting to compile here using GCC 4.9.2 results in
[Z:\Mozprofiles\Downloads]gcc -Zomf vatest.c
[Z:\Mozprofiles\Downloads]dir vatest*
The volume label in drive Z is mozprofiles.
The Volume Serial Number is E5C3:E5BD.
Directory of Z:\Mozprofiles\Downloads
6-03-19 12:05p 453 0 a--- vatest.c
6-03-19 12:13p 4,904 0 a--- vatest.exe
2 file(s) 5,357 bytes used
20,105,801 K bytes free
[Z:\Mozprofiles\Downloads]vatest.exe
Sum of 10, 20 and 30 = 60
Sum of 4, 20, 25 and 30 = 79
Why aren't you installing GCC 4.9.2 with YUM/RPM or ANPM?