pkg-config file doesn't reflect `64_` symbol suffix

[rgommers]

If you build OpenBLAS for ILP64 with the standard symbol suffix 64_

$ make INTERFACE64=1 SYMBOLSUFFIX=64_ USE_OPENMP=0 NO_STATIC=1 BINARY="64" DYNAMIC_ARCH=1 QUIET_MAKE=1
$ make INTERFACE64=1 SYMBOLSUFFIX=64_ USE_OPENMP=0 NO_STATIC=1 BINARY="64" DYNAMIC_ARCH=1 QUIET_MAKE=1 PREFIX=$PWD/tmpdir install
$ ls tmpdir/lib/
cmake  libopenblas64_p-r0.3.21.dev.so  libopenblas64_.so  libopenblas64_.so.0  pkgconfig
$ ls tmpdir/lib/pkgconfig/
openblas.pc
$ cat tmpdir/lib/pkgconfig/openblas.pc 
libdir=/home/rgommers/code/tmp/OpenBLAS/tmpdir/lib
includedir=/home/rgommers/code/tmp/OpenBLAS/tmpdir/include
openblas_config= USE_64BITINT= DYNAMIC_ARCH=1 DYNAMIC_OLDER= NO_CBLAS= NO_LAPACK= NO_LAPACKE= NO_AFFINITY=1 USE_OPENMP=0 SKYLAKEX MAX_THREADS=24
version=0.3.21.dev
extralib=-lm -lpthread -lm -lpthread
suffix=64_
Name: openblas
Description: OpenBLAS is an optimized BLAS library based on GotoBLAS2 1.13 BSD version
Version: ${version}
URL: https://github.com/xianyi/OpenBLAS
Libs: -L${libdir} -lopenblas
Libs.private: ${extralib}
Cflags: -I${includedir}

If the library produced is libopenblas64_.so, then the link like should use -lopenblas64_ (and I tested that that fixes the issue I was seeing). Or more generally, it should probably contain -lopenblas${suffix} with a suffix variable defined above it.

The thing I'm less sure about is: should the produced .pc file be named openblas64_.pc instead of plain openblas.pc? Or perhaps better, don't use the symbol suffix in the name but name it openblas-ilp64.pc. That way it's actually possible to install both LP64 and ILP64 builds next to each other without a conflict, and select the one you want and know what it is without resorting to checking the binary itself via compiling some code. I'll also note that MKL uses a -lp64/-ilp64 scheme.

[martin-frbg]

Possibly broken by 2965 a while ago, ironically with the expressed purpose of simplyfying side-by-side installs
Nope, broken even before that (also USE64BITINT should not stay unset in the .pc) cmake builds probably correct

[martin-frbg]

So the library suffixing appears to have been broken (in the gmake build system) forever, while the cmake build does it right. (But both were not reporting the setting of INTERFACE64 a.k.a USE64BITINT). I am not entirely sure what to use for the pkgconfig file suffix in the "ILP64" case - as far as I could tell, MKL is the only example that uses ilp64/lp64, while distribution packagers of OpenBLAS appear to have been using "openblas64.pc" already (which is also what our cmake build does, and the PR now adopts is for gmake builds as well)

[rgommers]

Then openblas64.pc should work great. Thanks for looking into it @martin-frbg!