Fix base_build() with a proper support for multi-package-building through conda-build

This MR fixes build.base_build() to properly handle building multiple versions of the same package according to what is defined in conda_build_config.yaml.

conda/conda_build_config.yaml

+macos_min_version:
+  - 10.9
+macos_machine:
+  - x86_64-apple-darwin13.4.0
+MACOSX_DEPLOYMENT_TARGET:
+  - 10.9
+CONDA_BUILD_SYSROOT:            # [osx]
+  - /opt/MacOSX10.9.sdk         # [osx]
+# This helps CMAKE find the sysroot. See
+# https://cmake.org/cmake/help/v3.11/variable/CMAKE_OSX_SYSROOT.html
+SDKROOT:                        # [osx]
+  - /opt/MacOSX10.9.sdk         # [osx]
+# makes autotools verbose
+VERBOSE_AT:
+  - V=1
+# makes cmake verbose
+VERBOSE_CM:
+  - VERBOSE=1
+
+## the dependencies that we build against multiple versions
+python:
+  - 3.6
+  - 3.7
+  - 3.8