CMake: correct typos, compiler checks and message type

- Fix variable name typo in `buildinfo.cmake`.
- Use `CMAKE_CXX_COMPILER_ID` instead of `CMAKE_C_COMPILER_ID`
  when adding C++ compiler flags in `gflags` and `gtest`.
- Change message type from `ERROR` to `STATUS` for missing system Python
  in tests configuration, matching similar messages that aren't fatal.

build_files/cmake/buildinfo.cmake

@@ -82,7 +82,7 @@ if(EXISTS ${SOURCE_DIR}/.git)
       endif()
 
       unset(_git_tag_hashes)
-      unset(_git_head_hashs)
+      unset(_git_head_hash)
     endif()
 
 

extern/gflags/CMakeLists.txt

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 # Too noisy for code we don't maintain.
-if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   add_cxx_flag("-Wno-cast-function-type")
 endif()
 

extern/gtest/CMakeLists.txt

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 # avoid noisy warnings
-if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   remove_cc_flag(
     "-Wmissing-declarations"
   )

tests/python/CMakeLists.txt

@@ -30,9 +30,8 @@ endif()
 
 if(WITH_SYSTEM_PYTHON_TESTS)
   if(NOT EXISTS "${TEST_SYSTEM_PYTHON_EXE}")
-    message(ERROR
-      "'System Python' tests requested but no valid system python path, "
-      "set TEST_SYSTEM_PYTHON_EXE."
+    message(STATUS
+      "Tests: Disabling 'System Python', no valid system python path, set TEST_SYSTEM_PYTHON_EXE."
     )
     set(WITH_SYSTEM_PYTHON_TESTS OFF)
   endif()