Substitute PEAR_INSTALLDIR only when enabling PEAR (#13994)

When not using --with-pear configure option, the Makefile doesn't need
the PEAR_INSTALLDIR variable. Also, this variable isn't directly
substituted using the @PEAR_INSTALLDIR@ in any template, so the
PHP_SUBST can be used.

configure.ac

@@ -1227,6 +1227,7 @@ if test "$PHP_PEAR" != "no"; then
     esac
   fi
 
+  PHP_SUBST([PEAR_INSTALLDIR])
   PHP_ADD_BUILD_DIR([pear])
   PHP_ADD_MAKEFILE_FRAGMENT($abs_srcdir/pear/Makefile.frag,$abs_srcdir/pear,pear)
 fi
@@ -1553,7 +1554,6 @@ PHP_SUBST_OLD(INSTALL_IT)
 PHP_SUBST(LIBTOOL)
 PHP_SUBST(LN_S)
 PHP_SUBST_OLD(NATIVE_RPATHS)
-PHP_SUBST_OLD(PEAR_INSTALLDIR)
 PHP_SUBST_OLD(PHP_LDFLAGS)
 PHP_SUBST(OVERALL_TARGET)
 PHP_SUBST(PHP_RPATHS)