Function and variable names beginning with __ are reserved for package manager use. Should be trivial to fix by replacing it by a single underscore. Policy reference: https://projects.gentoo.org/pms/8/pms.html#x1-13700012.3.17
This __GLIBC_CC is also in sys-libs/glibc ebuilds. Exporting of this variable (__GLIBC_CC or _GLIBC_CC) is probably unnecesary, i.e. you can remove it from export line.
Removed.