fix build with reentrant qhull

https://salsa.debian.org/science-team/plplot/-/blob/master/debian/patches/reentrant-qhull.patch

Index: cmake/modules/FindQHULL.cmake
--- cmake/modules/FindQHULL.cmake.orig
+++ cmake/modules/FindQHULL.cmake
@@ -16,27 +16,27 @@ include(FindPackageHandleStandardArgs)
 # that is what is supplied by upstream qhull, and if the user is
 # finding a local install of the upstream library below, he wants to
 # access that same upstream version of the headers to be consistent.
-find_file(QHULL_HEADER_FILE NAMES libqhull/qhull_a.h qhull/qhull_a.h)
+find_file(QHULL_HEADER_FILE NAMES libqhull_r/qhull_ra.h)
 if(QHULL_HEADER_FILE)
   # message(STATUS "QHULL_HEADER_FILE = ${QHULL_HEADER_FILE}")
-  if(QHULL_HEADER_FILE MATCHES "libqhull")
+  if(QHULL_HEADER_FILE MATCHES "libqhull_r")
     set(HAS_LIBQHULL_INCLUDE ON)
-  else(QHULL_HEADER_FILE MATCHES "libqhull")
+  else()
     set(HAS_LIBQHULL_INCLUDE OFF)
-  endif(QHULL_HEADER_FILE MATCHES "libqhull")
+  endif()
   # Determine grandparent directory of header file
   get_filename_component(QHULL_INCLUDE_DIRS ${QHULL_HEADER_FILE} DIRECTORY)
   get_filename_component(QHULL_INCLUDE_DIRS ${QHULL_INCLUDE_DIRS} DIRECTORY)
 elseif(NOT QHULL_HEADER_FILE AND NOT QHULL_FIND_QUIETLY)
-  message(STATUS "Neither qhull/qhull_a.h header nor libqhull/qhull_a.h could be found")
+  message(STATUS "libqhull_r/qhull_ra.h could not be found")
 endif(QHULL_HEADER_FILE)
 
-find_library(QHULL_LIBRARY qhull)
+find_library(QHULL_LIBRARY qhull_r)
 if( QHULL_LIBRARY )
   get_filename_component(QHULL_LIBRARY_DIRS ${QHULL_LIBRARY} DIRECTORY)
   set(QHULL_LIBRARIES ${QHULL_LIBRARY})
 elseif(NOT QHULL_LIBRARY AND NOT QHULL_FIND_QUIETLY)
-  message(STATUS "qhull library could not be found")
+  message(STATUS "qhull_r library could not be found")
 endif(QHULL_LIBRARY)
 
 find_package_handle_standard_args(QHULL