CMake: use more conventional checks that libraries were found

The method of checking the library was found cause an undefined
variable warning when it was found.

build_files/cmake/Modules/FindBrotli.cmake

@@ -59,7 +59,7 @@ FIND_LIBRARY(BROTLI_LIBRARY_DEC
 )
 
 
-IF(${BROTLI_LIBRARY_COMMON_NOTFOUND} or ${BROTLI_LIBRARY_DEC_NOTFOUND})
+IF((NOT BROTLI_LIBRARY_COMMON) OR (NOT BROTLI_LIBRARY_DEC))
   set(BROTLI_FOUND FALSE)
 ELSE()
   # handle the QUIETLY and REQUIRED arguments and set BROTLI_FOUND to TRUE if

build_files/cmake/Modules/FindUSD.cmake

@@ -52,7 +52,7 @@ FIND_LIBRARY(USD_LIBRARY
   DOC "Universal Scene Description (USD) monolithic library"
 )
 
-IF(${USD_LIBRARY_NOTFOUND})
+IF(NOT USD_LIBRARY)
   set(USD_FOUND FALSE)
 ELSE()
   # handle the QUIETLY and REQUIRED arguments and set USD_FOUND to TRUE if

build_files/cmake/Modules/FindWebP.cmake

@@ -59,7 +59,7 @@ FOREACH(COMPONENT ${_webp_FIND_COMPONENTS})
   LIST(APPEND _webp_LIBRARIES "${WEBP_${UPPERCOMPONENT}_LIBRARY}")
 ENDFOREACH()
 
-IF(${WEBP_WEBP_LIBRARY_NOTFOUND})
+IF(NOT WEBP_WEBP_LIBRARY)
   set(WEBP_FOUND FALSE)
 ELSE()
   # handle the QUIETLY and REQUIRED arguments and set WEBP_FOUND to TRUE if