[Octave-bug-tracker] [bug #43220] libgnu/math.h conflicting declaration of C function 'int signbit(double)'

[anonymous]

Follow-up Comment #2, bug #43220 (project octave):

I'm seeing this issue with 4.0.0-rc4 and suitesparse 4.3.x even though bug
#43063 is closed.  Can this be reopened?

More details:  I'm thinking there is a problem with detecting if signbit
exists, so it falls back to gnulib's implementation which causes more
problems.

checking for signbit macro... yes
checking for signbit compiler built-ins... yes
...
checking for std::signbit in <cmath>... yes
checking for std::signbit (float variant) in <cmath>... yes
...
checking for signbit... no
...
checking whether signbit is declared... yes


I'm wondering if the last two are necessary.  Here is the one that failed:

configure:70761: checking for signbit
configure:70761: gcc -o conftest -O2 -fPIC -pthread -fopenmp   conftest.c -lm 
 >&5   
conftest.c:845:6: warning: conflicting types for built-in function 'signbit'
[enabled by default]
 char signbit ();
      ^
/tmp/ccoCkqt6.o: In function `main':
conftest.c:(.text.startup+0x7): undefined reference to `signbit'
collect2: error: ld returned 1 exit status

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