cmake build: Add option to build against system SyntaxHighlighting lib

Enable/disable our own KSyntaxHighlighting by default
depending on the availability of KF5SyntaxHighlighting.
If that is available, it still is possible to turn the building
and using of our own KSyntaxHighlighting copy on by enabling
BUILD_LIBRARY_KSYNTAXHIGHLIGHTING.

Fixes: QTCREATORBUG-25076
Change-Id: I3545fc12d87572d7eb6f5977994b081ff7ea3880
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

CMakeLists.txt

@@ -53,6 +53,7 @@ find_package(Qt5 COMPONENTS Designer DesignerComponents Help SerialPort Svg Tool
 
 find_package(Threads)
 find_package(Clang QUIET)
+find_package(KF5SyntaxHighlighting QUIET)
 
 function (set_if_target var target)
    if (TARGET "${target}")

src/libs/3rdparty/syntax-highlighting/CMakeLists.txt

@@ -1,4 +1,11 @@
+if(TARGET KF5::SyntaxHighlighting)
+  set(HIGHLIGHTING_BUILD_DEFAULT OFF)
+else()
+  set(HIGHLIGHTING_BUILD_DEFAULT ON)
+endif()
+
 add_qtc_library(KSyntaxHighlighting SHARED
+  BUILD_DEFAULT ${HIGHLIGHTING_BUILD_DEFAULT}
   INCLUDES autogenerated/
   PUBLIC_INCLUDES
     src/lib

src/plugins/texteditor/CMakeLists.txt

@@ -1,6 +1,12 @@
+if(TARGET KSyntaxHighlighting)
+  set(KSYNTAXHIGHLIGHTING_TARGET KSyntaxHighlighting)
+else()
+  set(KSYNTAXHIGHLIGHTING_TARGET KF5::SyntaxHighlighting)
+endif()
+
 add_qtc_plugin(TextEditor
   DEPENDS Qt5::Concurrent Qt5::Network Qt5::PrintSupport Qt5::Xml
-  PUBLIC_DEPENDS KSyntaxHighlighting
+  PUBLIC_DEPENDS ${KSYNTAXHIGHLIGHTING_TARGET}
   PLUGIN_DEPENDS Core
   SOURCES
     autocompleter.cpp autocompleter.h