It may not apply clearly (currently on top of my -Wno-overloaded/autoconf-archive patch) but it should be easy enough to patch manually our configure ac,

Do not worry about patch applying cleanly as I always check it by hand too

re-ran ./autogen.shand see if it now correctly complains about lack of python if you remove it for a moment or ran on fresh OS  install without python3 yet?

The good news is that it does not cause any problem for my regular laptop Fedora OR ubuntu 16 (runs python 2.7.12) OR debian 9.1 32 bit (runs python 2.7.13).

The bad news is that it does not seem to work when I remove python from /usr/bin (actually I renamed python, python3 and python3.13 - so all of them/links) on another random system running Fedora 42.  First, I verified really gone by typing "python --version" and I get "command not found".  I do not see the "Please install python" in the log but see:

checking for libtool... $(SHELL) $(top_builddir)/libtool
checking for python3... no
checking for python... no
checking for gtk_init in -lm... yes

...cd openjpeg* && echo 'cmake "$@" "."' > ./configure; chmod +x ./configure;  ./configure -DBUILD_SHARED_LIBS:BOOL=OFF -DBUILD_CODEC:BOOL=OFF
patch -d mjpegtools* -p1 < src/mjpegtools-2.2.1.patch8
make[3]: Entering directory '/tmp/cinelerra-5.1/thirdparty/dav1d-0.5.1'
env: ‘python’: No such file or directory

...

CONFIGURING libjpeg
echo >> build/config.h "#define ARCH_ARM 0"
env: chmod 755 choose
‘python’: No such file or directory

In the end the build "errors out".

This does not fix check in guicast/Makefile for now, just hopefully warn user about potential troubles ahead if no python/python3 installed yet.

Put if check works as intended I hopefully can plumb it to guicast/Makefile.

Sorry for asking for such minor detail, but apparently I can't uninstall python in Termux, and my desktop is currently

No problem asking for such minor detail -- that is best to do. 

not powered yet. {having a bit of toothache in last few days, not great for concentration }

Hope the toothache gets better soon.