Add CMake option USE_BUNDLED_BLAS_WIN (#1940)

This option controls whether CMake uses the bundled OpenBLAS library
when building on Windows (defaults to True). Having this option allows a
user to build qiskit-aer against a different system BLAS without needing
to specify a full path using the `AER_BLAS_LIB_PATH` option.

Co-authored-by: Jun Doi <doichan@jp.ibm.com>

CMakeLists.txt

@@ -78,6 +78,8 @@ endif()
 option(STATIC_LINKING "Specify if we want statically link the executable (for
 						redistribution mainly)" FALSE)
 option(BUILD_TESTS "Specify whether we want to build tests or not" FALSE)
+option(USE_BUNDLED_BLAS_WIN "Use the bundled openblas library on Windows when
+							not using AER_BLAS_LIB_PATH" TRUE)
 
 # Allow disabling conan for downstream package managers. Requires all libraries to be present in path
 # Default is value of environment variable if defined or ON
@@ -237,7 +239,7 @@ else()
 		  set(BLA_VENDOR "OpenBLAS")
 		endif()
 	endif()
-	if(WIN32)
+	if(WIN32 AND USE_BUNDLED_BLAS_WIN)
 		message(STATUS "Uncompressing OpenBLAS static library...")
 		set(WIN_ARCH "win64" )
 		if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "4") # Checking if win32 build

CONTRIBUTING.md

@@ -966,6 +966,16 @@ These are the flags:
     Default: No value.
     Example: ``python ./setup.py bdist_wheel -- -DAER_BLAS_LIB_PATH=/path/to/look/for/blas/``
 
+* USE_BUNDLED_BLAS_WIN
+
+    Tells CMake to use the bundled OpenBLAS library vendored into the source code when building on Windows.
+    When this option is set to `False`, CMake will use its standard method to search for the BLAS library aginst which to link instead of using the vendored version.
+    The `AER_BLAS_LIB_PATH` option takes precedence over this option.
+
+    Values: True|False
+    Default: True
+    Example: ``python ./setup.py bdist_wheel -- -DUSE_BUNDLED_BLAS_WIN=FALSE``
+
 * BUILD_TESTS
 
     It will tell the build system to build C++ tests along with the simulator.

releasenotes/notes/windows-blas-42b3dbc170c5ae94.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Added CMake option ``USE_BUNDLED_BLAS_WIN`` which allows CMake to search
+    for a BLAS library against which to link instead of using the OpenBLAS
+    library bundled into the repo.