Link against older, more-compatible versiosn of glibc symbols.

They're binary-compatible, the old versions are more widely available
and the new versions are more highly optimized. We prefer the former.

.github/workflows/autobuild.yaml

@@ -62,7 +62,7 @@ jobs:
         - name: linux
           os: ubuntu-latest
           preinstall: sudo apt-get install ninja-build libz-dev
-          cflags: -O3 -gline-tables-only -DNDEBUG
+          cflags: -O3 -gline-tables-only -DNDEBUG -include $GITHUB_WORKSPACE/.github/workflows/lib_compat.h
           cmake: >
             "-DCMAKE_C_COMPILER=clang"
             "-DCMAKE_CXX_COMPILER=clang++"

.github/workflows/lib_compat.h

@@ -0,0 +1,18 @@
+// This header forces the use of a particular version of glibc symbols.
+// This can make the resulting binary portable to systems with older glibcs.
+//
+// It must be included in each TU, with CFLAGS="-include glibc_compat.h" etc.
+//
+// We only list symbols known to be used. This is paired with a test enforcing
+// the max glibc symbol version. If the test fails, the list should be extended.
+// Find the old alternatives version to target with e.g.:
+//   objdump -T /lib/x86_64-linux-gnu/lib{c,m,pthread,rt}.so.* | grep "\bexpf\b"
+
+#define FORCE_SYMBOL_VERSION(sym, version) \
+  __asm__(".symver " #sym "," #sym "@" #version)
+
+FORCE_SYMBOL_VERSION(expf, GLIBC_2.2.5);
+FORCE_SYMBOL_VERSION(log, GLIBC_2.2.5);
+FORCE_SYMBOL_VERSION(pow, GLIBC_2.2.5);
+
+#undef FORCE_SYMBOL_VERSION