Merge branch 'pw/use-glibc-tunable-for-malloc-optim' into maint

Avoid repeatedly running getconf to ask libc version in the test
suite, and instead just as it once per script.
source: <[email protected]>

* pw/use-glibc-tunable-for-malloc-optim:
  tests: cache glibc version check

Author: gitster

t/test-lib.sh

@@ -557,14 +557,19 @@ then
 		: nothing
 	}
 else
+	_USE_GLIBC_TUNABLES=
+	if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
+	   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
+	   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
+	then
+		_USE_GLIBC_TUNABLES=YesPlease
+	fi
 	setup_malloc_check () {
 		local g
 		local t
 		MALLOC_CHECK_=3	MALLOC_PERTURB_=165
 		export MALLOC_CHECK_ MALLOC_PERTURB_
-		if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
-		   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
-		   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
+		if test -n "$_USE_GLIBC_TUNABLES"
 		then
 			g=
 			LD_PRELOAD="libc_malloc_debug.so.0"