meson: Make detection of python more robust

Previously we errored out if no python installation could be found (but we did
handle not having enough of python installed to build plpython
against). Presumably nobody hit this so far, as python is likely installed due
to meson requiring python.

Author: Tristan Partin <tristan@neon.tech>
Discussion: https://postgr.es/m/CSPIJVUDZFKX.3KHMOAVGF94RV@c3po
Backpatch: 16-, where meson support was added

meson.build

@@ -1056,15 +1056,17 @@ endif
 ###############################################################
 
 pyopt = get_option('plpython')
+python3_dep = not_found_dep
 if not pyopt.disabled()
   pm = import('python')
   python3_inst = pm.find_installation(required: pyopt)
-  python3_dep = python3_inst.dependency(embed: true, required: pyopt)
-  if not cc.check_header('Python.h', dependencies: python3_dep, required: pyopt)
-    python3_dep = not_found_dep
+  if python3_inst.found()
+    python3_dep = python3_inst.dependency(embed: true, required: pyopt)
+    # Remove this check after we depend on Meson >= 1.1.0
+    if not cc.check_header('Python.h', dependencies: python3_dep, required: pyopt)
+      python3_dep = not_found_dep
+    endif
   endif
-else
-  python3_dep = not_found_dep
 endif