Improve robustness of Makefiles at finding some external programs (#996)

This adds a couple of robustness improvements to Makefiles to help
portability to different environments:

* If `python3-config` is not found, try to look for `python3.XY-config`,
where `XY` is the minor version of the Python in the host environment.
This is necessary because in at least some OS/Python vesion combos,
`python3-config` is not defined by installing the `python3-dev` system
package, and is named `python3.XY-config` instead.

* `wget` is not a standard utility in many Linux distros, Sometimes
`curl` is available. If `wget` is not found, try `curl` before giving
up.

Author: mhucka

Makefile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# If this is changed, updated the value in ./WORKSPACE too.
+# If you change the following values, update the values in ./WORKSPACE too.
 EIGEN_COMMIT = "b66188b5dfd147265bfa9ec47595ca0db72d21f5"
 EIGEN_URL = "https://gitlab.com/libeigen/eigen/-/archive/"
 
@@ -94,7 +94,7 @@ else
         ifneq (,$(strip $(wildcard $(CUDA_PATH)/.)))
             # $CUDA_PATH is set, but we know we didn't find nvcc on the user's
             # $PATH or as an absolute path (if $NVCC was set to a full path).
-	    # Try the safest choice for finding nvcc & give up if that fails.
+            # Try the safest choice for finding nvcc & give up if that fails.
             NVCC = $(CUDA_PATH)/bin/nvcc
             ifneq (,$(strip $(wildcard $(NVCC))))
                 CXXFLAGS += -I$(CUDA_PATH)/include -L$(CUDA_PATH)/lib64
@@ -120,7 +120,7 @@ ifneq (,$(strip $(CUQUANTUM_ROOT)))
         TARGETS += qsim-custatevecex
         TESTS += run-custatevec-tests
         TESTS += run-custatevecex-tests
-	TESTS += run-custatevecex-mpi-tests
+    TESTS += run-custatevecex-mpi-tests
     else
         $(warning $$CUQUANTUM_ROOT is set, but the path does not seem to exist)
     endif
@@ -218,9 +218,18 @@ check-cuquantum-root-set:
 	    exit 1; \
 	fi
 
+# Check whether wget or curl is available on this system.
+ifneq (,$(shell command -v wget))
+    DOWNLOAD_CMD := wget
+else ifneq (,$(shell command -v curl))
+    DOWNLOAD_CMD := curl --fail -L -O
+else
+    $(error Neither wget nor curl found in PATH. Please install curl or wget.)
+endif
+
 eigen:
 	-rm -rf eigen
-	wget $(EIGEN_URL)/$(EIGEN_COMMIT)/eigen-$(EIGEN_COMMIT).tar.gz
+	$(DOWNLOAD_CMD) $(EIGEN_URL)/$(EIGEN_COMMIT)/eigen-$(EIGEN_COMMIT).tar.gz
 	tar -xzf eigen-$(EIGEN_COMMIT).tar.gz && mv eigen-$(EIGEN_COMMIT) eigen
 	rm eigen-$(EIGEN_COMMIT).tar.gz
 

pybind_interface/Makefile

@@ -12,7 +12,16 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-SUFFIX := $(shell python3-config --extension-suffix)
+PYTHON_CONFIG := $(shell command -v python3-config)
+ifeq (,$(PYTHON_CONFIG))
+    PY_VER := $(shell python3 --version 2>&1 | awk -F'[ .]' '{print $$2"."$$3}')
+    PYTHON_CONFIG := $(shell command -v python$(PY_VER)-config)
+    ifeq (,$(PYTHON_CONFIG))
+        $(error Could not find python3-config or python$(PY_VER)-config.)
+    endif
+endif
+
+SUFFIX := $(shell $(PYTHON_CONFIG) --extension-suffix)
 
 # The names of the shared libraries that result after compiling qsim for Pybind11
 QSIMLIB_BASIC = ../qsimcirq/qsim_basic$(SUFFIX)