druntime integration tests: Fix musl detection with dash shell

As dash apparently doesn't understand the `&>` redirection, see
ldc-developers/ldc#4970 (comment),
causing IS_MUSL to be wrongly set on non-musl distros.

Author: kinke

druntime/test/exceptions/Makefile

@@ -1,7 +1,9 @@
-ifeq ($(OS),linux)
-    # FIXME: detect musl libc robustly; just checking Alpine Linux' apk tool for now
-    ifeq (1,$(shell which apk &>/dev/null && echo 1))
-        IS_MUSL:=1
+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
 

druntime/test/importc_compare/Makefile

@@ -1,11 +1,18 @@
 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 ($(OS),linux)
-    ifeq (1,$(shell which apk &>/dev/null && echo 1))
-        TESTS :=
-    endif
+ifeq ($(IS_MUSL),1)
+    TESTS :=
 endif
 
 include ../common.mak