You need to install gstreamer development tools as well
pacman -S mingw-w64-x86_64-gst-devtools
Now I am stuck here, seems like a problem in qwt_painter_command.h
compiling C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_date.cpp
compiling C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_date_scale_draw.cpp
compiling C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_date_scale_engine.cpp
compiling C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_dyngrid_layout.cpp
compiling C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_event_pattern.cpp
compiling C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_graphic.cpp
In file included from C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_graphic.cpp:11:
C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_painter_command.h:85:22: error: field 'clipPath' has incomplete type 'QPainterPath'
85 | QPainterPath clipPath;
| ^~~~~~~~
In file included from C:/msys64/mingw64/include/QtGui/qtransform.h:43,
from C:/msys64/mingw64/include/QtGui/qimage.h:48,
from C:/msys64/mingw64/include/QtGui/qpixmap.h:49,
from C:/msys64/mingw64/include/QtGui/qpainter.h:48,
from C:/msys64/mingw64/include/QtGui/qpaintengine.h:47,
from C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_null_paintdevice.h:15,
from C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_graphic.h:14,
from C:/msys64/home/admin/librepilot/ground/gcs/src/libs/qwt/src/qwt_graphic.cpp:10:
C:/msys64/mingw64/include/QtGui/qmatrix.h:54:7: note: forward declaration of 'class QPainterPath'
54 | class QPainterPath;
| ^~~~~~~~~~~~
mingw32-make[5]: *** [Makefile:1271: qwt_graphic.o] Error 1
mingw32-make[5]: Leaving directory 'C:/msys64/home/admin/librepilot/build/librepilot-gcs_release/src/libs/qwt/src'
mingw32-make[4]: *** [Makefile:50: sub-src-make_first-ordered] Error 2
mingw32-make[4]: Leaving directory 'C:/msys64/home/admin/librepilot/build/librepilot-gcs_release/src/libs/qwt'
mingw32-make[3]: *** [Makefile:317: sub-qwt-make_first-ordered] Error 2
mingw32-make[3]: Leaving directory 'C:/msys64/home/admin/librepilot/build/librepilot-gcs_release/src/libs'
mingw32-make[2]: *** [Makefile:53: sub-libs-make_first-ordered] Error 2
mingw32-make[2]: Leaving directory 'C:/msys64/home/admin/librepilot/build/librepilot-gcs_release/src'
mingw32-make[1]: *** [Makefile:52: sub-src-make_first-ordered] Error 2
mingw32-make[1]: Leaving directory 'C:/msys64/home/admin/librepilot/build/librepilot-gcs_release'
mingw32-make: *** [Makefile:308: gcs] Error 2