[lapack-reference] Cannot disable blas-select on Linux

[cgmb]

Describe the bug
On Linux, USE_OPTIMIZED_BLAS=ON appears to always be set when using an external Fortran compiler, but it should only be true when using the blas-select or noblas features.

Environment

• OS: Linux (Ubuntu 18.04)
• Compiler: gfortran

To Reproduce
I created a project to illustrate the problem. It sets "default-features": false to disable blas-select.

Steps to reproduce the behavior:

1. Download the example project: https://gist.github.com/cgmb/371a8279ee5bc8f63285d0b96d920a41
2. Configure the project.

cmake -S. -Bbuild -DCMAKE_TOOLCHAIN_FILE=$VCPKG_PATH/scripts/buildsystems/vcpkg.cmake -DVCPKG_TARGET_TRIPLET=x64-linux

3. Examine the lapack-reference build parameters:

grep USE_OPTIMIZED_BLAS $VCPKG_PATH/buildtrees/lapack-reference/x64-linux-rel/CMakeCache.txt

Actual Output:

USE_OPTIMIZED_BLAS:BOOL=ON

Expected Output:

USE_OPTIMIZED_BLAS:BOOL=OFF

[PhoebeHui]

@cgmb,t thanks for reporting this issue! Could you help test linked PR and see if it works for you?

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