Trac #29448: configure finds libpng but matplotlib does not

This happens at least in the following situation: Sage 9.1.beta9 on OS
X, using the system's rather than Sage's Python, no pkg-config
installed.

URL: https://trac.sagemath.org/29448
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

build/pkgs/libpng/spkg-configure.m4

@@ -8,9 +8,13 @@ SAGE_SPKG_CONFIGURE([libpng], [
       AC_MSG_RESULT([no])
       dnl First try checking for libpng with pkg-config
       PKG_CHECK_MODULES([LIBPNG], [libpng >= 1.2], [], [
-        dnl Fallback to manually grubbing around for headers and libs
-        AC_CHECK_HEADERS([png.h], [break], [sage_spkg_install_libpng=yes])
-        AC_SEARCH_LIBS([png_get_io_ptr], [png], [], [sage_spkg_install_libpng=yes])
+        sage_spkg_install_libpng=yes
+        dnl Yes, we *could* fallback to manually grubbing around for headers and libs as follows:
+        dnl   AC_CHECK_HEADERS([png.h], [break], [sage_spkg_install_libpng=yes])
+        dnl   AC_SEARCH_LIBS([png_get_io_ptr], [png], [], [sage_spkg_install_libpng=yes])
+        dnl But 'matplotlib' and 'sagelib' rely on pkg-config to locate libpng.
+        dnl So we would have to tell them about the libpng that we found,
+        dnl for example by creating a facade .pc file like we do for BLAS.
       ])
     fi
 ])