Fix GLSLANG_ENABLE_INSTALL when SHADERC_SKIP_INSTALL is ON

[rootkiller6788]

Problem

Running cmake with SHADERC_SKIP_INSTALL=ON fails during generation:

CMake Error in third_party/glslang/CMakeLists.txt:
  install(EXPORT "glslang-targets" ...) includes target "SPIRV" which
  requires target "SPIRV-Tools-opt" that is not in any export set.

Root cause

third_party/CMakeLists.txt sets

set(GLSLANG_ENABLE_INSTALL $<NOT:${SKIP_GLSLANG_INSTALL}>)

Generator expressions are not evaluated at configure time. When SHADERC_SKIP_INSTALL is ON, the variable holds the literal string $<NOT:ON>, which CMake's if() treats as truthy, so glslang still enables its install rules. Since SPIRV-Tools' install is skipped in that configuration, glslang's install(EXPORT glslang-targets) references targets (SPIRV, glslang) whose dependency SPIRV-Tools-opt is not in any export set, and generation fails.

Fix

Compute the boolean directly instead of relying on a generator expression:

if (SKIP_GLSLANG_INSTALL)
  set(GLSLANG_ENABLE_INSTALL OFF)
else()
  set(GLSLANG_ENABLE_INSTALL ON)
endif()

Verification

• cmake -DSHADERC_SKIP_INSTALL=ON ... previously failed with the error above; with the fix it configures successfully and glslang's cmake_install.cmake contains no install rules.
• The default configuration (without SHADERC_SKIP_INSTALL) still configures successfully and glslang's install rules are unchanged.

Fixes #1435

[YGXXD]

I encountered the same problem.