libc/glibc: set the float option according to extra CFLAGS

When building a multilib, some extra CFLAGS can override the
default config option. This is the case for the floating point
selection.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 This is NOT the official version of dialog.  This version has been

 significantly modified from the original.  It is for use by the Linux

 kernel configuration script.  Please do not bother Savio Lam with 

 questions about this program.