diff --git a/ax_check_qt.m4 b/ax_check_qt.m4
index 88dc733..211513a 100644
--- a/ax_check_qt.m4
+++ b/ax_check_qt.m4
@@ -42,10 +42,10 @@ AC_DEFUN([AX_CXX_QT_TOOL], [
     HAVE_$1=1
     AC_MSG_CHECKING([for $2])
     AC_ARG_VAR([$1], [path to Qt tool $2])
-    $1=${$1:-$(${ac_tool_prefix}pkg-config --variable=$2_location Qt5Core)}
-    $1=${$1:-$(${ac_tool_prefix}pkg-config --variable=host_bins Qt5Core)/$2}
-    $1=${$1:-$(${ac_tool_prefix}pkg-config --variable=$2_location QtCore)}
-    $1=${$1:-$(${ac_tool_prefix}pkg-config --variable=host_bins QtCore)/$2}
+    $1=${$1:-$(${PKG_CONFIG} --variable=$2_location Qt5Core)}
+    $1=${$1:-$(${PKG_CONFIG} --variable=host_bins Qt5Core)/$2}
+    $1=${$1:-$(${PKG_CONFIG} --variable=$2_location QtCore)}
+    $1=${$1:-$(${PKG_CONFIG} --variable=host_bins QtCore)/$2}
     if ! which "${$1%% *}" > /dev/null; then
       if which "$2" > /dev/null; then
         $1=$2
@@ -100,7 +100,7 @@ AC_DEFUN([AX_CHECK_QT], [
   PKG_CHECK_MODULES([$1]5, [${qt_modules//Qt/Qt5}], [
     HAVE_$1=1
     AC_DEFINE([HAVE_$1])
-    QTDIR=$(${ac_tool_prefix}pkg-config --variable=prefix Qt5Core)
+    QTDIR=$(${PKG_CONFIG} --variable=prefix Qt5Core)
     QT_PLUGIN_PATH=${QTDIR}/share/qt5/plugins
     CPPFLAGS+=" ${[$1]5_CFLAGS}"
     MOC_FLAGS+=" -DHAVE_$1=1 ${[$1]5_CFLAGS}"
@@ -122,7 +122,7 @@ AC_DEFUN([AX_CHECK_QT], [
     PKG_CHECK_MODULES([$1], [${qt_modules}], [
       HAVE_$1=1
       AC_DEFINE([HAVE_$1])
-      QTDIR=$(${ac_tool_prefix}pkg-config --variable=prefix QtCore)
+      QTDIR=$(${PKG_CONFIG} --variable=prefix QtCore)
       QT_PLUGIN_PATH=${QTDIR}/share/qt/plugins
       CPPFLAGS+=" ${$1_CFLAGS}"
       MOC_FLAGS+=" -DHAVE_$1=1 ${$1_CFLAGS}"
diff --git a/ax_init_standard_project.m4 b/ax_init_standard_project.m4
index a900722..87ad90c 100644
--- a/ax_init_standard_project.m4
+++ b/ax_init_standard_project.m4
@@ -438,7 +438,7 @@ AC_DEFUN([AX_PKG_REQUIRE], [
         fi
       done
       if test "${$1_found}" = "no"; then
-        tmp_includedir=$(${ac_tool_prefix}pkg-config --variable=includedir $tmp_package)
+        tmp_includedir=$(${PKG_CONFIG} --variable=includedir $tmp_package)
         for x in ${tmp_includedir}; do
           AC_MSG_NOTICE([search for $3 in $x])
           for f in $(find ${x} -name "$3"); do