FindPostgreSQL: support version encoding used in pre-10 releases

With the 10.x release, PostgreSQL upstream started encoding the version
as `MMmmmm` where `M` is major and `m` is minor. Prior to that, `MMmmPP`
was used where `P` was the patch number. Detect this difference and
decode it based on the used encoding.

Fixes: #19912

Modules/FindPostgreSQL.cmake

@@ -208,11 +208,22 @@ if (PostgreSQL_INCLUDE_DIR)
     endif()
   endforeach()
   if (_PostgreSQL_VERSION_NUM)
-    math(EXPR _PostgreSQL_major_version "${_PostgreSQL_VERSION_NUM} / 10000")
-    math(EXPR _PostgreSQL_minor_version "${_PostgreSQL_VERSION_NUM} % 10000")
-    set(PostgreSQL_VERSION_STRING "${_PostgreSQL_major_version}.${_PostgreSQL_minor_version}")
-    unset(_PostgreSQL_major_version)
-    unset(_PostgreSQL_minor_version)
+    # 9.x and older encoding
+    if (_PostgreSQL_VERSION_NUM LESS 100000)
+      math(EXPR _PostgreSQL_major_version "${_PostgreSQL_VERSION_NUM} / 10000")
+      math(EXPR _PostgreSQL_minor_version "${_PostgreSQL_VERSION_NUM} % 10000 / 100")
+      math(EXPR _PostgreSQL_patch_version "${_PostgreSQL_VERSION_NUM} % 100")
+      set(PostgreSQL_VERSION_STRING "${_PostgreSQL_major_version}.${_PostgreSQL_minor_version}.${_PostgreSQL_patch_version}")
+      unset(_PostgreSQL_major_version)
+      unset(_PostgreSQL_minor_version)
+      unset(_PostgreSQL_patch_version)
+    else ()
+      math(EXPR _PostgreSQL_major_version "${_PostgreSQL_VERSION_NUM} / 10000")
+      math(EXPR _PostgreSQL_minor_version "${_PostgreSQL_VERSION_NUM} % 10000")
+      set(PostgreSQL_VERSION_STRING "${_PostgreSQL_major_version}.${_PostgreSQL_minor_version}")
+      unset(_PostgreSQL_major_version)
+      unset(_PostgreSQL_minor_version)
+    endif ()
   else ()
     foreach(_PG_CONFIG_HEADER ${_PG_CONFIG_HEADERS})
       if(EXISTS "${_PG_CONFIG_HEADER}")

Tests/FindPostgreSQL/Test/main.c

@@ -5,10 +5,19 @@
 int main()
 {
   int version = PQlibVersion();
-  int major = version / 10000;
-  int minor = version % 10000;
   char version_string[100];
-  snprintf(version_string, sizeof(version_string), "%d.%d", major, minor);
+  // 9.x and older encoding.
+  if (version < 100000) {
+    int major = version / 10000;
+    int minor = version % 10000 / 100;
+    int patch = version % 100;
+    snprintf(version_string, sizeof(version_string), "%d.%d.%d", major, minor,
+             patch);
+  } else {
+    int major = version / 10000;
+    int minor = version % 10000;
+    snprintf(version_string, sizeof(version_string), "%d.%d", major, minor);
+  }
   printf("Found PostgreSQL version %s, expected version %s\n", version_string,
          CMAKE_EXPECTED_POSTGRESQL_VERSION);
   return strcmp(version_string, CMAKE_EXPECTED_POSTGRESQL_VERSION);