Fix compile error on Fedora 40

Signed-off-by: David Abdurachmanov <davidlt@rivosinc.com>

mp.spec

@@ -34,7 +34,7 @@
 
 Name: mp
 Version: 3.1.0
-Release: 42.%{date}git%{shortcommit}%{?dist}
+Release: 42.%{date}git%{shortcommit}.0.riscv64%{?dist}
 License: MIT and BSD
 Summary: An open-source library for mathematical programming
 URL: https://github.com/ampl/mp
@@ -184,6 +184,10 @@ sed -i 's,returncode == 0,False,' support/build-docs.py
 %endif
 
 %build
+# Fedora 40 compile issues
+CFLAGS="-D_GNU_SOURCE $CFLAGS"
+CXXFLAGS="-D_GNU_SOURCE $CXXFLAGS"
+
 export LIBS="-lgsl -l%{blaslib}"
 mkdir -p build && pushd build
 BUILD="asl,gsl,smpswriter"
@@ -314,6 +318,9 @@ popd
 %doc build/doc/ampl.github.io/*
 
 %changelog
+* Fri Dec 08 2023 David Abdurachmanov <davidlt@rivosinc.com> - 3.1.0-42.20200303git7fd4828.0.riscv64
+- Fix compile error on Fedora 40
+
 * Thu Jul 20 2023 Fedora Release Engineering <releng@fedoraproject.org> - 3.1.0-42.20200303git7fd4828
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_39_Mass_Rebuild