xbps-src: add numpy build_helper

Packages that link against python3-numpy require special consideration
to ensure that, in a cross-building environment, target libraries and
headers as well as the FORTRAN compiler and linker are properly
identifeid. Previously, this was done directly in the python3-scipy
template to support cross compilation. Now, a build_helper generalizes
these changes to support other package templates.

common/build-helper/numpy.sh

@@ -0,0 +1,36 @@
+#
+# numpy - build-helper for packages that compile against python3-numpy
+#
+# This build-helper makes sure packages can find python3-numpy libraries and
+# headers on the target architecture rather than the host, as well as making
+# sure the gfortran cross compiler is properly identified.
+
+# Even for cross compilation, numpy should be available on the host to ensure
+# that the host interpreter doesn't complain about missing deps
+if [[ $hostmakedepends != *"python3-numpy"* ]]; then
+	hostmakedepends+=" python3-numpy"
+fi
+
+if [ "$CROSS_BUILD" ]; then
+	if [[ $makedepends != *"python3-numpy"* ]]; then
+		makedepends+=" python3-numpy"
+	fi
+
+	# python3-setuptools finds numpy libs and headers on the host first;
+	# addding search paths up front allows the target to take priority
+	CFLAGS+=" -I${XBPS_CROSS_BASE}/${py3_sitelib}/numpy/core/include"
+	LDFLAGS+=" -L${XBPS_CROSS_BASE}/${py3_sitelib}/numpy/core/lib"
+
+	# distutils from python3-numpy looks to environment variables F77 and
+	# F90 rather than the XBPS-set FC
+	export F77="${FC}"
+	export F90="${FC}"
+
+	# When compiling and linking FORTRAN, distutils from python3-numpy
+	# refuses respect any linker name except "gfortran"; symlink to the
+	# cross-compiler to that the right linker and compiler will be used
+	if _gfortran=$(command -v "${FC}"); then
+		ln -sf "${_gfortran}" "${XBPS_WRAPPERDIR}/gfortran"
+	fi
+	unset _gfortran
+fi