Improve PkgConfig usage

Including FindPkgConfig directly into scripts is not a recommended practice.
Doing so will cause CMake to complain with a warning.
Replaced with find_package(PkgConfig QUIET)

CMakeModules/FindGTA.cmake

@@ -8,14 +8,10 @@
 # correspond to the ./configure --prefix=$GTA_DIR
 # used in building libgta.
 
-INCLUDE(FindPkgConfig OPTIONAL)
+find_package(PkgConfig QUIET)
 
 IF(PKG_CONFIG_FOUND)
-
-    INCLUDE(FindPkgConfig)
-
     PKG_CHECK_MODULES(GTA gta)
-
 ENDIF(PKG_CONFIG_FOUND)
 
 IF(NOT GTA_FOUND)
@@ -54,4 +50,4 @@ IF(NOT GTA_FOUND)
         SET(GTA_FOUND "YES")
     ENDIF(GTA_LIBRARIES AND GTA_INCLUDE_DIRS)
 
-ENDIF(NOT GTA_FOUND)
+ENDIF(NOT GTA_FOUND)

CMakeModules/FindGtkGl.cmake

@@ -1,10 +1,7 @@
 #use pkg-config to find various modues
-INCLUDE(FindPkgConfig OPTIONAL)
+find_package(PkgConfig QUIET)
 
 IF(PKG_CONFIG_FOUND)
-
-    INCLUDE(FindPkgConfig)
-
     PKG_CHECK_MODULES(GTK gtk+-2.0)
 
     IF(WIN32)

CMakeModules/FindPoppler-glib.cmake

@@ -1,10 +1,7 @@
 #use pkg-config to find various modues
-INCLUDE(FindPkgConfig OPTIONAL)
+find_package(PkgConfig QUIET)
 
 IF(PKG_CONFIG_FOUND)
-
-    INCLUDE(FindPkgConfig)
-
     PKG_CHECK_MODULES(CAIRO cairo)
     PKG_CHECK_MODULES(POPPLER poppler-glib)
 

CMakeModules/FindRSVG.cmake

@@ -1,10 +1,7 @@
 #use pkg-config to find various modues
-INCLUDE(FindPkgConfig OPTIONAL)
+find_package(PkgConfig QUIET)
 
 IF(PKG_CONFIG_FOUND)
-
-    INCLUDE(FindPkgConfig)
-
     #Version 2.35 introduces the rsvg_cleanup function which is used
     PKG_CHECK_MODULES(RSVG librsvg-2.0>=2.35)
 
@@ -14,5 +11,4 @@ IF(PKG_CONFIG_FOUND)
        SET(RSVG_FOUND FALSE)
     ENDIF()
 
-
 ENDIF()

src/osgViewer/CMakeLists.txt

@@ -185,7 +185,7 @@ ELSE()
 
     ELSEIF(${OSG_WINDOWING_SYSTEM} STREQUAL "X11")
         # X11 for everybody else
-        INCLUDE(FindPkgConfig OPTIONAL)
+        find_package(PkgConfig QUIET)
         IF(PKG_CONFIG_FOUND)
 
             PKG_CHECK_MODULES(XRANDR xrandr)