Current implementation requires vsx: https://github.com/freebsd/freebsd-ports/commit/d442c49fd6131a5aef6597ec739ac46ccdf2c5a7
It also uses incorrect macros and non-portable ways to detect ISA support.
We fix macros and add an explicit option to use VSX.

--- CMakeLists.txt	2026-04-22 05:01:59.000000000 +0800
+++ CMakeLists.txt	2026-05-29 15:14:11.000000000 +0800
@@ -37,6 +37,7 @@
 option(SIMDUTF_INTERNAL_TESTS "Whether to test also internal procedures. Useful mostly for developers, not users." OFF)
 option(SIMDUTF_LOGGING "Whether to enable logging (this should never be used in binary releases)." OFF)
 option(SIMDUTF_USE_STATIC_INITIALIZATION "Whether to use translation-unit-scope static variables for implementation singletons (faster, but unsafe before main() when used in a library)." OFF)
+option(SIMDUTF_ENABLE_VSX "Use VSX instructions on supported Power CPUs." OFF)
 option(FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION "Whether to enable unsafe fuzzing mode." OFF)
 
 set(SIMDUTF_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})

--- src/CMakeLists.txt	2026-04-22 05:01:59.000000000 +0800
+++ src/CMakeLists.txt	2026-05-29 15:30:30.000000000 +0800
@@ -51,7 +51,9 @@
   target_compile_options(simdutf PRIVATE -mno-avx256-split-unaligned-load -mno-avx256-split-unaligned-store)
 endif()
 
-if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU") AND (CMAKE_SYSTEM STREQUAL "PowerPC"))
+# VSX may or may not be available, do not force possibly unsupported flags.
+if (CMAKE_SYSTEM_PROCESSOR MATCHES "ppc|power" AND SIMDUTF_ENABLE_VSX)
+  target_compile_definitions(simdutf PRIVATE SIMDUTF_ENABLE_VSX=1)
   target_compile_options(simdutf PRIVATE -maltivec -mvsx -mcpu=power8)
 endif()
 
--- src/simdutf/ppc64.h	2026-04-22 05:01:59.000000000 +0800
+++ src/simdutf/ppc64.h	2026-05-29 15:28:17.000000000 +0800
@@ -8,10 +8,10 @@
 #include "simdutf/portability.h"
 
 #ifndef SIMDUTF_IMPLEMENTATION_PPC64
-  #define SIMDUTF_IMPLEMENTATION_PPC64 (SIMDUTF_IS_PPC64)
+  #define SIMDUTF_IMPLEMENTATION_PPC64 (SIMDUTF_PPC64_VSX)
 #endif
 #define SIMDUTF_CAN_ALWAYS_RUN_PPC64                                           \
-  SIMDUTF_IMPLEMENTATION_PPC64 &&SIMDUTF_IS_PPC64
+  SIMDUTF_IMPLEMENTATION_PPC64 && SIMDUTF_PPC64_VSX
 
 #include "simdutf/internal/isadetection.h"
 
--- include/simdutf/portability.h	2026-04-22 05:01:59.000000000 +0800
+++ include/simdutf/portability.h	2026-05-29 15:28:00.000000000 +0800
@@ -109,9 +109,10 @@
   #define SIMDUTF_IS_X86_64 1
 #elif defined(__aarch64__) || defined(_M_ARM64) || defined(_M_ARM64EC)
   #define SIMDUTF_IS_ARM64 1
-#elif defined(__PPC64__) || defined(_M_PPC64)
-  #if defined(__VEC__) && defined(__ALTIVEC__)
-    #define SIMDUTF_IS_PPC64 1
+#elif defined(__PPC64__) || defined(_M_PPC64) || defined(__ppc64__)
+  #define SIMDUTF_IS_PPC64 1
+  #if defined(__VEC__) && defined(SIMDUTF_ENABLE_VSX)
+    #define SIMDUTF_PPC64_VSX 1
   #endif
 #elif defined(__s390__)
 // s390 IBM system. Big endian.
@@ -177,7 +178,7 @@
     #define SIMDUTF_IS_X86_32BITS 1
   #elif defined(__arm__) || defined(_M_ARM)
     #define SIMDUTF_IS_ARM_32BITS 1
-  #elif defined(__PPC__) || defined(_M_PPC)
+  #elif defined(__PPC__) || defined(_M_PPC) || defined(__ppc__)
     #define SIMDUTF_IS_PPC_32BITS 1
   #endif