Use `pkg_get_variable` to retrieve the value of pkg-config variable

It is a new feature in CMake version 3.4, CMake-3.5 is required for
TDE now.

Signed-off-by: OBATA Akio <obache@wizdas.com>
(cherry picked from commit 6feea3aea9)

ConfigureChecks.cmake

@@ -26,11 +26,10 @@ endif( WITH_GCC_VISIBILITY )
 if( BUILD_TQCA )
 ##### look for tqt3 headers path
 
-execute_process(
-      COMMAND ${PKG_CONFIG_EXECUTABLE}
-      --variable=includedir tqt-mt
-      OUTPUT_VARIABLE TQT_HEADERS_DIRS
-      OUTPUT_STRIP_TRAILING_WHITESPACE
+pkg_get_variable(
+      TQT_HEADERS_DIRS
+      tqt-mt
+      includedir
 )
 endif( BUILD_TQCA )
 
@@ -66,11 +65,10 @@ endif()
 
 ##### look for tqt3 plugins path
 
-execute_process(
-      COMMAND ${PKG_CONFIG_EXECUTABLE}
-      --variable=pluginsdir tqt-mt
-      OUTPUT_VARIABLE TQT_PLUGINS_DIR
-      OUTPUT_STRIP_TRAILING_WHITESPACE
+pkg_get_variable(
+      TQT_PLUGINS_DIR
+      tqt-mt
+      pluginsdir
 )
 if( TQT_PLUGINS_DIR )
     set( TQT_PLUGINS_CRYPTO_DIR "${TQT_PLUGINS_DIR}/crypto" )