Backport of upstream commit 1c246c2e91fa (part of "Code cleanup (#692)"): only
build the VSX dispatcher when the compiler supports VSX. On big-endian
powerpc64 the VSX helper is unavailable, so the dispatcher referenced
Sleef_*vsx functions that were never built and libsleef.so could not be
loaded (undefined symbol Sleef_sind2_u35vsx).

--- src/libm/CMakeLists.txt.orig	2026-10-09 21:23:09 UTC
+++ src/libm/CMakeLists.txt
@@ -790,7 +790,7 @@ endif(SLEEF_ARCH_X86)
   endif()
 endif(SLEEF_ARCH_X86)
 
-if (SLEEF_ARCH_PPC64)
+if (SLEEF_ARCH_PPC64 AND COMPILER_SUPPORTS_VSX)
   # Target disppower_128.c
   sleef_generate_disp_simd(power_128 2 4 SLEEF_VECTOR_DOUBLE SLEEF_VECTOR_FLOAT SLEEF_VECTOR_INT vsx vsx3 x)
 
@@ -815,7 +815,7 @@ if (SLEEF_ARCH_PPC64)
     set_target_properties(tryvsx3_obj PROPERTIES ${COMMON_TARGET_PROPERTIES})
     target_sources(${TARGET_LIBSLEEF} PRIVATE $<TARGET_OBJECTS:tryvsx3_obj>)
   endif()
-endif(SLEEF_ARCH_PPC64)
+endif(SLEEF_ARCH_PPC64 AND COMPILER_SUPPORTS_VSX)
 
 if (SLEEF_ARCH_S390X)
   # Target disps390x_128.c