Handle POSIX shell for tests in configure and Makefile

Although the trick of finding a POSIX shell in the system PATH works
fine most of the time, it has some drawbacks.

* The commands must be copied into every test script.
* The scripts are always forced to re-execute themselves.
* There's no guarantee the sh found in `getconf PATH` is a POSIX shell
  and there's no way to override it.

Move the handling of this shell to configure where we can detect it
once. This gives preference to bash and ksh since they're typically
POSIX compatible. It also uses the current PATH with the getconf PATH at
the end which should allow things to work on platforms where getconf
might not be available like mingw/msys.

By specifying the shell in TESTS_ENVIRONMENT, automake will run each
script with this shell and we can drop the re-exec dance.

check/Makefile.am

@@ -1,3 +1,5 @@
+TESTS_ENVIRONMENT = $(TESTS_SHELL)
+
 TESTS = \
 	check-cflags \
 	check-libs \

check/check-cflags

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-cmd-options

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-conflicts

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-define-variable

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-idirafter

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-includedir

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-libs

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-libs-private

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-missing

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-requires-private

@@ -1,9 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
 set -e
 
 . ${srcdir}/common

check/check-version

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

check/check-whitespace

@@ -1,10 +1,5 @@
 #! /bin/sh
 
-# Make sure we're POSIX
-if [ "$PKG_CONFIG_SHELL_IS_POSIX" != "1" ]; then
-    PKG_CONFIG_SHELL_IS_POSIX=1 PATH=`getconf PATH` exec sh $0 "$@"
-fi
-
 set -e
 
 . ${srcdir}/common

configure.ac

@@ -23,6 +23,13 @@ AC_PROG_CC
 dnl Check for headers
 AC_CHECK_HEADERS([dirent.h unistd.h sys/wait.h malloc.h])
 
+dnl A POSIX shell is required for the tests. If TEST_SHELL hasn't been
+dnl set on the command line then we try to find bash or ksh or sh from
+dnl the path. If none of those are available, we just use whatever
+dnl autoconf detected for configure.
+AC_ARG_VAR([TESTS_SHELL], [Path to a POSIX shell to be used for testing])
+conf_path="$PATH:`getconf PATH 2>/dev/null`"
+AC_PATH_PROGS([TESTS_SHELL], [bash ksh sh], [$CONFIG_SHELL], [$conf_path])
 
 dnl
 dnl Default pkg-config search path