MF stable/12 r352685:

libm: Include float.h to get LDBL_MANT_DIG

The long double aliases of double functions are only exposed as aliases
if LDBL_MANT_DIG is 53 (same as DBL_MANT_DIG). Without float.h
included these files were not exposing weak aliases as expected,
leading to link failures if programs use the *l functions. This should
fix editors/calligra on targets with 64-bit long double, which uses
erfl and erfcl. Found on powerpc64.

Approved by: re(delphij)