game-music-emu: do not export symbols from C sources, i.e.: gme/ext/*.c

SDL Commits
1 
From 8387fede7aa9c111584fb5bbffc0d66364a7cc47 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Wed, 4 Oct 2023 14:50:00 +0300
Subject: [PATCH] do not export symbols from C sources, i.e.: gme/ext/*.c

---
 CMakeLists.txt | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6e0e288..1644db3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -77,15 +77,14 @@ if(NOT BUILD_SHARED_LIBS)
 endif()
 
 # Check for GCC/Clang "visibility" support.
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU"
-    OR
-    CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
 
     set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -W -Wextra")
     set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 
     # Assume we have visibility support on any compiler that supports C++11
     add_definitions (-DLIBGME_VISIBILITY)
+    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fvisibility=hidden")
     set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fvisibility=hidden -fvisibility-inlines-hidden")
 
     # Try to protect against undefined behavior from signed integer overflow