build: install glslang-config.cmake to libdir

build: install glslang-config.cmake to libdir

As glslang ships architecture dependant files, the Config file should be
installed to libdir, not datadir. See
https://github.com/KhronosGroup/glslang/pull/2989#discussion_r955367103
for more details.

Here's the diff between the install tree before and after this patch:

    $ diff <(tree install-datadir) <(tree install)
    1c1
    < install-datadir
    ---
    > install
    74,99c74,98
    <         ├── lib
    <         │   ├── cmake
    <         │   │   ├── glslang-default-resource-limitsTargets.cmake
    <         │   │   ├── glslangTargets.cmake
    <         │   │   ├── glslangValidatorTargets.cmake
    <         │   │   ├── HLSLTargets.cmake
    <         │   │   ├── OGLCompilerTargets.cmake
    <         │   │   ├── OSDependentTargets.cmake
    <         │   │   ├── spirv-remapTargets.cmake
    <         │   │   ├── SPIRVTargets.cmake
    <         │   │   └── SPVRemapperTargets.cmake
    <         │   ├── libGenericCodeGen.a
    <         │   ├── libglslang.a
    <         │   ├── libglslang-default-resource-limits.a
    <         │   ├── libHLSL.a
    <         │   ├── libMachineIndependent.a
    <         │   ├── libOGLCompiler.a
    <         │   ├── libOSDependent.a
    <         │   ├── libSPIRV.a
    <         │   └── libSPVRemapper.a
    <         └── share
    <             └── glslang
    <                 ├── glslang-config.cmake
    <                 ├── glslang-config-version.cmake
    <                 ├── glslang-targets.cmake
    <                 └── glslang-targets-debug.cmake
    ---
    >         └── lib
    >             ├── cmake
    >             │   ├── glslang-default-resource-limitsTargets.cmake
    >             │   ├── glslangTargets.cmake
    >             │   ├── glslangValidatorTargets.cmake
    >             │   ├── HLSLTargets.cmake
    >             │   ├── OGLCompilerTargets.cmake
    >             │   ├── OSDependentTargets.cmake
    >             │   ├── spirv-remapTargets.cmake
    >             │   ├── SPIRVTargets.cmake
    >             │   └── SPVRemapperTargets.cmake
    >             ├── glslang
    >             │   ├── glslang-config.cmake
    >             │   ├── glslang-config-version.cmake
    >             │   ├── glslang-targets.cmake
    >             │   └── glslang-targets-debug.cmake
    >             ├── libGenericCodeGen.a
    >             ├── libglslang.a
    >             ├── libglslang-default-resource-limits.a
    >             ├── libHLSL.a
    >             ├── libMachineIndependent.a
    >             ├── libOGLCompiler.a
    >             ├── libOSDependent.a
    >             ├── libSPIRV.a
    >             └── libSPVRemapper.a
    101c100
    < 15 directories, 83 files
    ---
    > 14 directories, 83 files