Why is C90 enforced in KDE?

[Antonio Rojas]

Hi,
 Kipi-plugins fails to build with flex 2.6. This is due to the autogenerated 
code in libpanorama containing //-style comments, which are disallowed in 
C90. Adding -std=c99 to the CFLAGS at compile time doesn't have any effect, 
since it is overriden by kdelibs (and by extra-cmake-modules in KF5). What 
is the reason for this? And is there any way to force using C99?