Link to system qcustomplot instead of bundled one.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -515,17 +515,12 @@ set (wsjtx_UISRCS
   messagereplydialog.ui
   )
 
-set (qcp_CXXSRCS
-  qcustomplot-source/qcustomplot.cpp
-  )
-
 set (all_CXXSRCS
   ${wsjt_CXXSRCS}
   ${wsjt_qt_CXXSRCS}
   ${wsjt_qtmm_CXXSRCS}
   ${js8_CXXSRCS}
   ${wsjtx_CXXSRCS}
-  ${qcp_CXXSRCS}
   )
 
 set (all_C_and_CXXSRCS
@@ -957,16 +952,11 @@ if (${OPENMP_FOUND} OR APPLE)
   endif (APPLE)
 endif (${OPENMP_FOUND} OR APPLE)
 
-# build a library for the QCustomPlot widget
-add_library (qcp STATIC ${qcp_CXXSRCS})
-target_include_directories (qcp PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/qcustomplot-source>)
-target_link_libraries (qcp Qt5::Widgets Qt5::PrintSupport)
-
 # build a library of package Qt functionality
 add_library (wsjt_qt STATIC ${wsjt_qt_CXXSRCS} ${wsjt_qt_GENUISRCS} ${GENAXSRCS})
 # set wsjtx_udp exports to static variants
 target_compile_definitions (wsjt_qt PUBLIC UDP_STATIC_DEFINE)
-target_link_libraries (wsjt_qt qcp Qt5::Widgets Qt5::Network)
+target_link_libraries (wsjt_qt qcustomplot Qt5::Widgets Qt5::Network)
 target_include_directories (wsjt_qt BEFORE PRIVATE ${hamlib_INCLUDE_DIRS})
 if (WIN32)
   target_link_libraries (wsjt_qt Qt5::AxContainer Qt5::AxBase)
