Improve pkg-config file generation for external BLAS/LAPACK dependencies

[zoziha]

Description

Problem Statement

When building the Fortran Standard Library with external BLAS/LAPACK libraries, the generated fortran_stdlib.pc file includes absolute paths to the BLAS/LAPACK libraries. This is problematic for system packaging (e.g., MSYS2) where the library should reference system-wide installed libraries using standard pkg-config syntax.

Current Behavior

• Absolute paths in pkg-config: The generated fortran_stdlib.pc file includes absolute paths to BLAS/LAPACK libraries, e.g., /path/to/libopenblas.dll or /path/to/mkl_core.dll
• No pkg-config integration: The build system does not leverage existing pkg-config files for BLAS/LAPACK libraries
• Packaging complexity: System packagers need to patch the generated pkg-config file

Root Cause

The current build system directly uses the absolute paths found by find_package(BLAS) and find_package(LAPACK) without considering if the BLAS/LAPACK vendor provides a pkg-config file.
(I know little about MKL and am not sure if it provides .pc files, while openblas has one.)

Proposed Solution

1. Detect pkg-config files: Check if common BLAS/LAPACK vendors (e.g., OpenBLAS, Intel MKL) provide pkg-config files
2. Use standard syntax: When a pkg-config file exists, use the Requires field instead of absolute paths
3. Maintain compatibility: Fall back to absolute paths when no pkg-config file is available

Expected Behaviour

Current problematic output:

Libs: -L${libdir} -lfortran_stdlib /path/to/libopenblas.dll ...

Desired output:

Requires: openblas
Libs: -L${libdir} -lfortran_stdlib ...

or just

Libs: -L${libdir} -lfortran_stdlib -lopenblas ...
Cflags: -I${includedir} -I${moduledir} -I/folder/to/libopenblas.dll

Version of stdlib

0ede301

Platform and Architecture

Windows OS

Additional Information

No response

[jvdp1]

When building the Fortran Standard Library with external BLAS/LAPACK libraries, the generated fortran_stdlib.pc file includes absolute paths to the BLAS/LAPACK libraries. This is problematic for system packaging (e.g., MSYS2) where the library should reference system-wide installed libraries using standard pkg-config syntax.

A (temporary?) solution could be to disable the support of external BLAS/LAPACK dependencies with FIND_BLAS (option(FIND_BLAS "Find external BLAS and LAPACK" ON)).

The current build system directly uses the absolute paths found by find_package(BLAS) and find_package(LAPACK) without considering if the BLAS/LAPACK vendor provides a pkg-config file. (I know little about MKL and am not sure if it provides .pc files, while openblas has one.)

MKL provides several pkg-config files (static/dynamic, sequential/gomp/iomp, ilp64,...).

Desired output:

Requires: openblas
Libs: -L${libdir} -lfortran_stdlib ...

or just

Libs: -L${libdir} -lfortran_stdlib -lopenblas ...
Cflags: -I${includedir} -I${moduledir} -I /folder/to/libopenblas.dll

Actually I think that the BLAS library should be mentioned in Libs.private and Requires.private