[reduce LDC-specific diff wrt. IS_MUSL]

Based on dlang/dmd#21779. For LDC, always
define IS_MUSL externally, not just to `1` for musl targets.

Author: kinke

runtime/DRuntimeIntegrationTests.cmake

@@ -57,7 +57,7 @@ else()
     list(REMOVE_ITEM testnames uuid)
 endif()
 
-set(musl "")
+set(musl "IS_MUSL=0")
 if(TARGET_SYSTEM MATCHES "musl")
     set(musl "IS_MUSL=1")
 endif()

runtime/druntime/test/exceptions/Makefile

@@ -3,6 +3,15 @@ ifdef IN_LDC
 include ../../../../dmd/osmodel.mak
 endif
 
+ifndef IS_MUSL # LDC defines it externally
+    ifeq ($(OS),linux)
+        # FIXME: detect musl libc robustly; just checking Alpine Linux' apk tool for now
+        ifeq (1,$(shell which apk >/dev/null 2>&1 && echo 1))
+            IS_MUSL := 1
+        endif
+    endif
+endif
+
 TESTS=stderr_msg unittest_assert invalid_memory_operation static_dtor \
       future_message refcounted rt_trap_exceptions_drt catch_in_finally \
       message_with_null

runtime/druntime/test/importc_compare/Makefile

@@ -5,12 +5,19 @@ endif
 
 TESTS := importc_compare
 
+ifndef IS_MUSL # LDC defines it externally
+    ifeq ($(OS),linux)
+        # FIXME: detect musl libc robustly; just checking Alpine Linux' apk tool for now
+        ifeq (1,$(shell which apk >/dev/null 2>&1 && echo 1))
+            IS_MUSL := 1
+        endif
+    endif
+endif
+
 # FIXME: fails on Alpine v3.21 with conflicting struct declarations in the C headers:
 # /usr/include/asm-generic/fcntl.h(195): Error: struct `importc_includes.flock` conflicts with struct `importc_includes.flock` at /usr/include/fcntl.h(24)
-ifeq ($(OS),linux)
-    ifeq (1,$(IS_MUSL))
-        TESTS :=
-    endif
+ifeq ($(IS_MUSL),1)
+    TESTS :=
 endif
 
 # FIXME: fails on macOS arm64, e.g., due to unsupported `_Float16`