Honor a pre-set SDCPP_BUILD_VERSION/SDCPP_BUILD_COMMIT so a release-tarball
build embeds the real version instead of "unknown". Upstream unconditionally
runs `git describe` in the source dir, which is not a git checkout when built
from a tarball, so its empty output overwrites the -D values the ebuild passes.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -240,7 +240,11 @@
 )
 
 find_program(GIT_EXE NAMES git git.exe NO_CMAKE_FIND_ROOT_PATH)
-if(GIT_EXE)
+# Gentoo: honor -DSDCPP_BUILD_VERSION / -DSDCPP_BUILD_COMMIT passed from the
+# ebuild so a release-tarball build embeds the real version instead of
+# "unknown". git describe runs in the source dir (not a git checkout here) and
+# would otherwise clobber the pre-set values with empty output.
+if(GIT_EXE AND NOT SDCPP_BUILD_VERSION)
     execute_process(COMMAND ${GIT_EXE} describe --tags --abbrev=7 --dirty=+
         WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
         OUTPUT_VARIABLE SDCPP_BUILD_VERSION