Deps: ensure osl/bin/oslc is using static libpng

Pass `-DLINKSTATIC=ON` to the OSL CMake, to ensure it statically links to
our libpng. Previously this was only applied on Windows, it's now on all
platforms.

build_files/build_environment/cmake/osl.cmake

@@ -20,12 +20,10 @@ if(WIN32)
   set(OSL_CMAKE_CXX_STANDARD_LIBRARIES "kernel32${LIBEXT} user32${LIBEXT} gdi32${LIBEXT} winspool${LIBEXT} shell32${LIBEXT} ole32${LIBEXT} oleaut32${LIBEXT} uuid${LIBEXT} comdlg32${LIBEXT} advapi32${LIBEXT} psapi${LIBEXT}")
   set(OSL_FLEX_BISON -DFLEX_EXECUTABLE=${LIBDIR}/flexbison/win_flex.exe -DBISON_EXECUTABLE=${LIBDIR}/flexbison/win_bison.exe)
   set(OSL_SIMD_FLAGS -DOIIO_NOSIMD=1 -DOIIO_SIMD=sse2)
-  SET(OSL_PLATFORM_FLAGS -DLINKSTATIC=ON)
 else()
   set(OSL_CMAKE_CXX_STANDARD_LIBRARIES)
   set(OSL_FLEX_BISON)
   set(OSL_OPENIMAGEIO_LIBRARY "${LIBDIR}/openimageio/lib/${LIBPREFIX}OpenImageIO${LIBEXT};${LIBDIR}/openimageio/lib/${LIBPREFIX}OpenImageIO_Util${LIBEXT};${LIBDIR}/png/lib/${LIBPREFIX}png16${LIBEXT};${LIBDIR}/jpg/lib/${LIBPREFIX}jpeg${LIBEXT};${LIBDIR}/tiff/lib/${LIBPREFIX}tiff${LIBEXT};${LIBDIR}/openexr/lib/${LIBPREFIX}IlmImf${OPENEXR_VERSION_POSTFIX}${LIBEXT}")
-  SET(OSL_PLATFORM_FLAGS)
 endif()
 
 set(OSL_ILMBASE_CUSTOM_LIBRARIES "${LIBDIR}/openexr/lib/Imath${OPENEXR_VERSION_POSTFIX}.lib^^${LIBDIR}/openexr/lib/Half{OPENEXR_VERSION_POSTFIX}.lib^^${LIBDIR}/openexr/lib/IlmThread${OPENEXR_VERSION_POSTFIX}.lib^^${LIBDIR}/openexr/lib/Iex${OPENEXR_VERSION_POSTFIX}.lib")
@@ -57,7 +55,7 @@ set(OSL_EXTRA_ARGS
   ${OSL_FLEX_BISON}
   -DCMAKE_CXX_STANDARD_LIBRARIES=${OSL_CMAKE_CXX_STANDARD_LIBRARIES}
   -DBUILD_SHARED_LIBS=OFF
-  ${OSL_PLATFORM_FLAGS}
+  -DLINKSTATIC=ON
   -DOSL_BUILD_PLUGINS=OFF
   -DSTOP_ON_WARNING=OFF
   -DUSE_LLVM_BITCODE=OFF