cmake: Fix cross-compilation on macOS

Co-authored-by: Owen Rudge <owen@owenrudge.net>
Co-authored-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>

CMakeLists.txt

@@ -16,6 +16,7 @@
 # Revised by Steve Robinson, 2020
 # Revised by Simon Hausmann, 2020
 # Revised by Alex Gaynor, 2020
+# Revised by Owen Rudge, 2020
 # Revised by Gleb Mazovetskiy, 2021
 
 # This code is released under the libpng license.
@@ -66,13 +67,24 @@ option(PNG_HARDWARE_OPTIMIZATIONS "Enable hardware optimizations" ON)
 set(PNG_PREFIX "" CACHE STRING "Prefix to add to the API function names")
 set(DFA_XTRA "" CACHE FILEPATH "File containing extra configuration settings")
 
+# CMake currently sets CMAKE_SYSTEM_PROCESSOR to one of x86_64 or arm64 on macOS,
+# based upon the OS architecture, not the target architecture. As such, we need
+# to check CMAKE_OSX_ARCHITECTURES to identify which hardware-specific flags to
+# enable. Note that this will fail if you attempt to build a universal binary in
+# a single CMake invocation.
+if (APPLE AND CMAKE_OSX_ARCHITECTURES)
+  set(TARGET_ARCH ${CMAKE_OSX_ARCHITECTURES})
+else()
+  set(TARGET_ARCH ${CMAKE_SYSTEM_PROCESSOR})
+endif()
+
 if(PNG_HARDWARE_OPTIMIZATIONS)
 
 # Set definitions and sources for ARM.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^arm" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "^aarch64")
-  if(CMAKE_SYSTEM_PROCESSOR MATCHES "^arm64" OR
-     CMAKE_SYSTEM_PROCESSOR MATCHES "^aarch64")
+if(TARGET_ARCH MATCHES "^arm" OR
+   TARGET_ARCH MATCHES "^aarch64")
+  if(TARGET_ARCH MATCHES "^arm64" OR
+     TARGET_ARCH MATCHES "^aarch64")
     set(PNG_ARM_NEON_POSSIBLE_VALUES on off)
     set(PNG_ARM_NEON "on"
         CACHE STRING "Enable ARM NEON optimizations: on|off; on is default")
@@ -103,8 +115,8 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^arm" OR
 endif()
 
 # Set definitions and sources for PowerPC.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^powerpc*" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "^ppc64*")
+if(TARGET_ARCH MATCHES "^powerpc*" OR
+   TARGET_ARCH MATCHES "^ppc64*")
   set(PNG_POWERPC_VSX_POSSIBLE_VALUES on off)
   set(PNG_POWERPC_VSX "on"
       CACHE STRING "Enable POWERPC VSX optimizations: on|off; on is default")
@@ -126,8 +138,8 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^powerpc*" OR
 endif()
 
 # Set definitions and sources for Intel.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^i?86" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "^x86_64*")
+if(TARGET_ARCH MATCHES "^i?86" OR
+   TARGET_ARCH MATCHES "^x86_64*")
   set(PNG_INTEL_SSE_POSSIBLE_VALUES on off)
   set(PNG_INTEL_SSE "on"
       CACHE STRING "Enable INTEL_SSE optimizations: on|off; on is default")
@@ -149,8 +161,8 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^i?86" OR
 endif()
 
 # Set definitions and sources for MIPS.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "mipsel*" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "mips64el*")
+if(TARGET_ARCH MATCHES "mipsel*" OR
+   TARGET_ARCH MATCHES "mips64el*")
   set(PNG_MIPS_MSA_POSSIBLE_VALUES on off)
   set(PNG_MIPS_MSA "on"
       CACHE STRING "Enable MIPS_MSA optimizations: on|off; on is default")
@@ -174,26 +186,26 @@ endif()
 else(PNG_HARDWARE_OPTIMIZATIONS)
 
 # Set definitions and sources for ARM.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^arm" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "^aarch64")
+if(TARGET_ARCH MATCHES "^arm" OR
+   TARGET_ARCH MATCHES "^aarch64")
   add_definitions(-DPNG_ARM_NEON_OPT=0)
 endif()
 
 # Set definitions and sources for PowerPC.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^powerpc*" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "^ppc64*")
+if(TARGET_ARCH MATCHES "^powerpc*" OR
+   TARGET_ARCH MATCHES "^ppc64*")
   add_definitions(-DPNG_POWERPC_VSX_OPT=0)
 endif()
 
 # Set definitions and sources for Intel.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^i?86" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "^x86_64*")
+if(TARGET_ARCH MATCHES "^i?86" OR
+   TARGET_ARCH MATCHES "^x86_64*")
   add_definitions(-DPNG_INTEL_SSE_OPT=0)
 endif()
 
 # Set definitions and sources for MIPS.
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "mipsel*" OR
-   CMAKE_SYSTEM_PROCESSOR MATCHES "mips64el*")
+if(TARGET_ARCH MATCHES "mipsel*" OR
+   TARGET_ARCH MATCHES "mips64el*")
   add_definitions(-DPNG_MIPS_MSA_OPT=0)
 endif()