build : use pkg-config for OpenBLAS (#1778)

build : use pkg-config for OpenBLAS (#1778)

* make : use pkg-config for finding CFLAGS & LDFLAGS needed by OpenBLAS

That way building on *nix like environments (including MSYS2 on Windows)
with WHISPER_OPENBLAS=1 works out of the box.

Fix handling of WHISPER_OPENBLAS, so that empty value or 0 won't be
misinterpreted by make as enabled.  Mind that it's not intended to
detect CMake false constants (OFF NO FALSE N).  make is not CMake.

By default OpenBLAS with 64-bit interface is used, but that can be
changed with `WHISPER_OPENBLAS_INTERFACE64=0` if 32-bit one is desired.

If OpenBLAS headers and library are respectively in include/ and lib/
subdirectories of given path, then you can specify it, e.g.
`OPENBLAS_PATH=/usr/local/openblas`, and this will take precedence over
any pkg-config file.

If there is no pkg-config file (.pc) for OpenBLAS and OPENBLAS_PATH is
empty, then headers are assumed to be in /usr/include/openblas and
library as assumed to be called 'openblas64' (or 'openblas' if
`WHISPER_OPENBLAS_INTERFACE64=0`).  If different headers location should
be used, then it can be done, e.g.
`WHISPER_BLAS_CFLAGS=-I/usr/local/include/openblas`.
If different library should be used, it can be specified, e.g.
`WHISPER_BLAS_LIB=openblasp64` (pthreads version as seen on Fedora), or
you can provide LDFLAGS needed to link with OpenBLAS directly:
`WHISPER_BLAS_LDFLAGS="-L/usr/local/lib/openblas -lopenblas64"`.

Current solution is flexible enough to handle most cases out there
without needlessly hardcoding possible OpenBLAS installation details.

* cmake : fix how pkg-config is used for finding include dirs and libraries needed by OpenBLAS

That way building on *nix like environments (including MSYS2 on Windows)
with -DWHISPER_OPENBLAS=ON should work out of the box as long as you
have CMake 3.25 or newer.

Make OPENBLAS_PATH environment variable supported not only on Windows.
It sets OpenBLAS include dir to ${OPENBLAS_PATH}/include and library to
${WHISPER_BLAS_LIB} (name without prefixes and suffixes) in
${OPENBLAS_PATH}/lib and avoids further package finding.

By default OpenBLAS with 64-bit interface is used (equivalent to setting
`-DWHISPER_BLAS_LIB=openblas64`), but that can be changed with
`-DWHISPER_OPENBLAS_INTERFACE64=OFF` (equivalent to setting
`-DWHISPER_BLAS_LIB=openblas`) if 32-bit one is desired.

Turn on BLA_STATIC for FindBLAS only when WHISPER_STATIC is enabled.
BLA_STATIC may not work as expected for pkg-config based operation.

Get rid of supporting BLAS_HOME environment variable.  If OPENBLAS_PATH
is insufficient in your case, there is no pkg-config file to rely on,
then you can manually specify include dir, e.g.
`-DBLAS_INCLUDE_DIRS=/usr/local/include/openblas`, and library, e.g.
`-DBLAS_LIBRARIES=/usr/local/lib/libopenblas.so`.

* make / cmake : use OpenBLAS with 32-bit interface by default.

OpenBLAS w/o INTERFACE64=1 vel USE_64BITINT=1 seems to be more common.

* cmake : hardcode "lib" prefix for OpenBLAS lib filename (even on Windows)

* cmake : hardcode OpenBLAS library name when building in MSVC (Windows)

Most *nix like environments (including MSYS2 on Windows) have OpenBLAS
packages that allow coexistence of OpenBLAS builds with 32-bit and
64-bit interface (w/o and w/ OPENBLAS_USE64BITINT defined) and they
differ by not having or having "64" suffix in their library filenames.
That's not the case for OpenBLAS prebuilt libraries for Windows.