configure.ac: Use AS_IF() instead of shell-if for PKG_CONFIG

https://autotools.io/pkgconfig/pkg_check_modules.html

> If you have the first call to PKG_CHECK_MODULES inside a bash
> conditional block, the expansion of PKG_PROG_PKG_CONFIG will also be
> conditional

> You can solve this problem in two ways; you can either explicitly call
> PKG_PROG_PKG_CONFIG outside of any conditional, forcing checking for
> pkg-config as soon as possible; or you can rewrite your conditionals
> to use the proper syntax
diff --git a/configure.ac b/configure.ac
index 126e9a5..74c266e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -89,9 +89,9 @@
     [
         enable_pcsc="yes"
     ])
-if test "x$enable_pcsc" = "xyes" ; then
+AS_IF([test "x$enable_pcsc" = "xyes"], [
 	PKG_CHECK_MODULES(PCSC, libpcsclite)
-fi
+])
 AM_CONDITIONAL(ENABLE_PCSC, test "x$enable_pcsc" = "xyes")
 
 AC_ARG_ENABLE(talloc,
@@ -109,12 +109,12 @@
 	)],
 	[enable_system_talloc=$enableval], [enable_system_talloc="no"])
 AM_CONDITIONAL(ENABLE_SYSTEM_TALLOC, [test x"$enable_system_talloc" = x"yes"])
-if test x"$enable_system_talloc" = x"yes"; then
+AS_IF([test x"$enable_system_talloc" = x"yes"], [
 	PKG_CHECK_MODULES(TALLOC, talloc)
 	if test x"$enable_talloc" = x"yes"; then
 		AC_MSG_ERROR([You cannot enable both internal and system talloc])
 	fi
-fi
+])
 
 AC_ARG_ENABLE(plugin,
 	[AS_HELP_STRING(