CMake: Check Git commit SHA1

This is for PandaSystem::get_git_commit()

dtool/PandaVersion.cmake

@@ -73,6 +73,16 @@ set(PANDA_VERSION_SYMBOL panda_version_${PROJECT_VERSION_MAJOR}_${PROJECT_VERSIO
 # for each component.
 math(EXPR PANDA_NUMERIC_VERSION "${PROJECT_VERSION_MAJOR}*1000000 + ${PROJECT_VERSION_MINOR}*1000 + ${PROJECT_VERSION_PATCH}")
 
+# The Panda Git SHA1 refspec, for PandaSystem::get_git_commit()
+find_package(Git QUIET)
+if(GIT_EXECUTABLE)
+  execute_process(
+    COMMAND "${GIT_EXECUTABLE}" rev-parse HEAD
+    WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
+    OUTPUT_VARIABLE PANDA_GIT_COMMIT_STR
+    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
+endif()
+
 # Separate the plugin version into its three components.
 string(REPLACE "." ";" P3D_PLUGIN_VERSION_LIST "${P3D_PLUGIN_VERSION}")
 list(GET P3D_PLUGIN_VERSION_LIST 0 P3D_PLUGIN_MAJOR_VERSION)

dtool/src/dtoolbase/pandaVersion.h.in

@@ -49,6 +49,10 @@
    out from CVS by the builder). */
 #define PANDA_VERSION_STR "@PANDA_VERSION_STR@"
 
+/* This is the Git commit we built Panda from, or an empty string if this isn't
+   known. */
+#define PANDA_GIT_COMMIT_STR "@PANDA_GIT_COMMIT_STR@"
+
 /* This is the version of the Panda3D ABI expressed as a string.
    This usually means the major and minor version. It should be the
    same for Panda3D versions that are supposed to be backward