cmake: locate wayland-scanner via find_program instead of pkg-config

particularly when cross compiling, we want to use the wayland-scanner
which is on the PATH of the build machine -- not the wayland-scanner
which will be on the target machine and could be for an architecture
we can't invoke during build.

cmake/ConkyPlatformChecks.cmake

@@ -408,6 +408,9 @@ if(BUILD_X11)
 endif(BUILD_X11)
 
 if(BUILD_WAYLAND)
+  # find 'wayland-scanner' executable
+  find_program(Wayland_SCANNER NAMES wayland-scanner)
+
   find_package(Wayland REQUIRED)
   set(conky_libs ${conky_libs} ${Wayland_CLIENT_LIBRARY})
   set(conky_includes ${conky_includes} ${Wayland_CLIENT_INCLUDE_DIR})
@@ -419,9 +422,6 @@ if(BUILD_WAYLAND)
   if(Wayland_FOUND AND wayland-protocols_FOUND)
     # find Wayland protocols
     pkg_get_variable(Wayland_PROTOCOLS_DIR wayland-protocols pkgdatadir)
-
-    # find 'wayland-scanner' executable
-    pkg_get_variable(Wayland_SCANNER wayland-scanner wayland_scanner)
   else(Wayland_FOUND AND wayland-protocols_FOUND)
     message(FATAL_ERROR "Unable to find wayland-scanner and xdg-shell protocol")
   endif(Wayland_FOUND AND wayland-protocols_FOUND)