The fundamental problem is that the glibc version is a string and not a decimal number. So for a "proper" solution you need to parse it manually and implement your own logic to decide which version is bigger or smaller.

However, as a quick and dirty hack, try inserting the line

setlocale(LC_NUMERIC, "C");

before the strtod call. That will set the numeric locale back to the default C locale where the decimal separator is .. If you're doing something that needs correct locales later in the program you need to set it back again. Depending on how your program initialized locales earlier, something like

setlocale(LC_NUMERIC, "");

should reset it back to what the environment says the locale should be.