[Bug libstdc++/60789] [4.9 Regression] Missing -lm while checking for math functions (e.g., atan2f)

[david.abdurachmanov at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60789

--- Comment #1 from David Abdurachmanov <david.abdurachmanov at gmail dot com> 
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Found the culprit. I had CFLAGS/CXXFLAGS/LDFLAGS for gcc ./configure. Thus
probably setting CFLAGS causes `-lm` to vanish in libstdc++v3 math conftests.