FindCBLAS.cmake: s/cblas/blas in pkg-config mode

[ghisvail]

BLAS detection does not work on Debian / Ubuntu due to the libblas-dev package shipping a blas.pc alongside headers and libraries and not a cblas.pc. This patch solves the detection.

[pavanky]

@ghisvail This was contributed by @marbre to work on Gentoo. Replacing cblas with blas may break builds on his package.

@marbre Can you check if simply using blas instead of cblas works on gentoo ? If so we could just accept the change. If not we should perhaps check for blas when cblas fails ?

[ghisvail]

It would not harm to check for both.

[marbre]

On Gentoo cblas and blas can be provided by different packages. For example the user can install blas-reference and cblas-reference. Another possibility is to install Atlas, which provides both. The user finally can choose which library to use with eselect {blas,cblas}.

Of cause, cblas requires a blas package installed, but not the other way around. Thus, checking for blas, it can not be guaranteed that cblas is available. Further cblas.pc and blas.pc pass back different Libs:

_cblas.pc_

prefix=/usr
libdir=${prefix}/lib64
includedir=${prefix}/include
Name: refcblas
Description: C wrapper interface to the F77 reference BLAS implementation
Version: 20110218
URL: http://www.netlib.org/blas/
Libs: -L${libdir} -lrefcblas
Private: -lm
Cflags: -I${includedir}/cblas
Requires: blas

_blas.pc_

prefix=/usr
libdir=/usr/lib64

Name: refblas
Description: Basic Linear Algebra Subprograms F77 reference implementations
Version: 
URL: http://www.netlib.org/refblas/
Libs: -L/usr/lib64 -lrefblas
Libs.private: -lm

I assume the cleanest way could be that Debian provides a blas.pc and a cblas.pc, with libblas-dev and libatlas-base-dev. But thats somehow a matter of taste.
If not, and if PKG_CHECK_MODULES(PC_CBLAS cblas) fails, the cblas can still be found with the other code in the FindCBLAS.cmake module, which I didn't touched. Thus, detecting cblas should also work on Debian without the patch, since I assume it worked before commit 644d074.

[pavanky]

@marbre thanks for the feedback.

@ghisvail I am on vacation and have limited time for testing. Do you mind maintaining this patch in your fork until we have a more permanent fix ?

[ghisvail]

@pavanky I am happy to hold on that patch. Afterall, it is distro specific.

On Gentoo cblas and blas can be provided by different packages.
Also the case on Arch apparently. Not the case on Debian and Fedora derivatives.

I assume the cleanest way could be that Debian provides a blas.pc and a cblas.pc.
Fair point, worth asking the Debian maintainers about this.

Since I don't want to break Gentoo's setup, I am now convinced that this patch should be kept downstream on Debian.

Feel free to close if you agree.