In order to install qscintilla we have to compile the code and install. Find the make install output from my windows below
ozkan@HP-ENVY-2021-I7 C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src
$ make install
C:/ProgramData/chocolatey/lib/make/tools/install/bin/make.exe -f Makefile.Release install
make[1]: Entering directory 'C:/Users/ozkan/tmp/QScintilla_src-2.14.1/src'
copy /y release\libqscintilla2_qt5.a C:\Qt\5.15.2\mingw81_64\lib\libqscintilla2_qt5.a
1 file(s) copied.
copy /y release\qscintilla2_qt5.dll C:\Qt\5.15.2\mingw81_64\lib\qscintilla2_qt5.dll
1 file(s) copied.
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src\Qsci C:\Qt\5.15.2\mingw81_64\include\Qsci
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src\qscintilla_cs.qm C:\Qt\5.15.2\mingw81_64\translations\qscintilla_cs.qm
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src\qscintilla_de.qm C:\Qt\5.15.2\mingw81_64\translations\qscintilla_de.qm
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src\qscintilla_es.qm C:\Qt\5.15.2\mingw81_64\translations\qscintilla_es.qm
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src\qscintilla_fr.qm C:\Qt\5.15.2\mingw81_64\translations\qscintilla_fr.qm
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src\qscintilla_pt_br.qm C:\Qt\5.15.2\mingw81_64\translations\qscintilla_pt_br.qm
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\qsci C:\Qt\5.15.2\mingw81_64\qsci
C:\Qt\5.15.2\mingw81_64\bin\qmake.exe -install qinstall C:\Users\ozkan\tmp\QScintilla_src-2.14.1\src\features\qscintilla2.prf C:\Qt\5.15.2\mingw81_64\mkspecs\features\qscintilla2.prf
make[1]: Leaving directory 'C:/Users/ozkan/tmp/QScintilla_src-2.14.1/src'
took like 10 minutes to compile in my machine. Downloaded from https://riverbankcomputing.com/software/qscintilla/download I am surprised in year of 2024 CPP does not have maven repo like structure. compiling the code is soo old now :)