libSDL2_image: configure behaves differently on Debian vs Ubuntu?

After almost a week of trying to figure this one out, I'm at a loss. 
This is maybe more of an autotools or distro-specific question, but 
perhaps someone here has more of a deeper understanding about what is 
happening and can help.

I am trying to build my own custom SDL2* libraries. I can configure and 
install SDL2 into an arbitrary directory just fine.

When I try to configure SDL2_image, pointing both the --prefix and 
--with-sdl-prefix to that same arbitrary directory, it succeeds just 
fine on Ubuntu.

But when I try to build on Debian Wheezy (mainly so my custom-built 
libraries, which I plan to distribute with my project, will have fewer 
dependencies than what Ubuntu pulls in), I am surprised to see the 
following output:

$ ./configure --disable-bmp --disable-gif --disable-jpg 
--disable-jpg-shared --disable-lbm --disable-pcx --enable-png 
--enable-png-shared --disable-pnm --disable-tga --disable-tif 
--disable-tif-shared --disable-xcf --disable-xpm --disable-xv 
--disable-webp --disable-webp-shared 
--prefix=/home/gb/Projects/Tools/CustomLibs/amd64 
--with-sdl-prefix=/home/gb/Projects/Tools/CustomLibs/amd64

...

checking for SDL - version >= 2.0.0... no
*** Could not run SDL test program, checking why...
*** The test program compiled, but did not run. This usually means
*** that the run-time linker is not finding SDL or finding the wrong
*** version of SDL. If it is not finding SDL, you'll need to set your
*** LD_LIBRARY_PATH environment variable, or edit /etc/ld.so.conf to point
*** to the installed location  Also, make sure you have run ldconfig if that
*** is required on your system
***
*** If you have an old version installed, it is best to remove it, although
*** you may also be able to get things to work by modifying LD_LIBRARY_PATH
configure: error: *** SDL version 2.0.0 not found!

Checking the Debian config.log, it looks like it is able to build 
conftest using SDL2, so configure clearly knows how to find it using 
sdl2-config, but when it tries to run conftest, the runtime linker 
doesn't know where to look to find SDL2.

Debian's config.log:
configure:12352: checking for SDL - version >= 2.0.0
configure:12440: gcc -o conftest -g -O2 
-I/home/gb/Projects/Tools/CustomLibs/amd64/include/SDL2 -D_REENTRANT 
conftest.c  -L/home/gb/Projects/Tools/CustomLibs/amd64/lib -lSDL2 >&5
configure:12440: $? = 0
configure:12440: ./conftest
./conftest: error while loading shared libraries: libSDL2-2.0.so.0: 
cannot open shared object file: No such file or directory
configure:12440: $? = 127
configure: program exited with status 127

Debian: $ ls /home/gb/Projects/Tools/CustomLibs/amd64/lib/
cmake  libSDL2-2.0.so.0  libSDL2-2.0.so.0.4.1  libSDL2.a  libSDL2.la 
libSDL2main.a  libSDL2.so  libSDL2_test.a  pkgconfig


On Ubuntu 14.04, I see that config.log has a few key output variables, 
such as LIBS, that point to the custom directory, whereas on Debian 
Wheezy, those same output variables are missing that information.

Ubuntu's config.log:
configure:12352: checking for SDL - version >= 2.0.0
configure:12440: gcc -o conftest -g -O2 
-I/home/gb/Projects/Tools/CustomLibs/amd64/include/SDL2 -D_REENTRANT 
conftest.c  -L/home/gb/Projects/Tools/CustomLibs/amd64/lib -lSDL2 >&5
configure:12440: $? = 0
configure:12440: ./conftest
configure:12440: $? = 0
configure:12455: result: yes

When I check the diffs between config.log files, I see how different 
these seemingly important values are:

< CFLAGS='-I/usr/include/libpng12   -g -O2 
-I/home/gb/Projects/Tools/CustomLibs/amd64/include/SDL2 -D_REENTRANT'
---
 > CFLAGS='-g -O2'


< SDL_CFLAGS='-I/home/gb/Projects/Tools/CustomLibs/amd64/include/SDL2 
-D_REENTRANT'
< SDL_LIBS='-L/home/gb/Projects/Tools/CustomLibs/amd64/lib -lSDL2'
---
 > SDL_CFLAGS=''
 > SDL_LIBS=''


I could always using LD_LIBRARY_PATH or do what the README.txt says 
about setting environment variables myself, but I am curious why running 
configure on one system without doing those things results in behavior 
that is different from the other system.

Any ideas?

Thanks for your time,
Gianfranco

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