$ gem install tlearn
# Building native extensions.  This could take a while...
# ERROR:  Error installing tlearn:
#   ERROR: Failed to build gem native extension.

#     ~/.rbenv/versions/2.1.2/bin/ruby extconf.rb
# creating Makefile

# make "DESTDIR=" clean

# make "DESTDIR="
# compiling activate.c
# activate.c:10:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
# #endif  EXP_TABLE
#         ^
#         //
# activate.c:19:1: warning: 'extern' ignored on this declaration [-Wmissing-declarations]
# extern  struct  cf {
# ^
# activate.c:28:1: warning: 'extern' ignored on this declaration [-Wmissing-declarations]
# extern  struct  nf {
# ^
# activate.c:42:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
# act_nds(aold,amem,anew,awt,local,atarget)
# ^~~~~~~
# activate.c:119:9: warning: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))'
#                                                                 exit(1);
#                                                                 ^
# activate.c:119:9: note: please include the header <stdlib.h> or explicitly provide a declaration for 'exit'
# activate.c:220:1: warning: control may reach end of non-void function [-Wreturn-type]
# }
# ^
#6 warnings generated.
# compiling arrays.c
# arrays.c:49:5: warning: implicit declaration of function 'free' is invalid in C99 [-Wimplicit-function-declaration]
#     free(error_values);
#     ^
# arrays.c:72:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
# make_arrays()
# ^~~~~~~~~~~
# arrays.c:84:3: warning: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))'
#                 exit(1);
#                 ^
# arrays.c:84:3: note: please include the header <stdlib.h> or explicitly provide a declaration for 'exit'
# arrays.c:124:60: warning: format specifies type 'int' but the argument has type 'unsigned long' [-Wformat]
#                 printf("wt malloc failed--needed %d bytes for pointers", nn*sizeof(float *));
#                                                  ~~                      ^~~~~~~~~~~~~~~~~~
#                                                  %lu
# arrays.c:197:1: warning: control may reach end of non-void function [-Wreturn-type]
# }
# ^
# arrays.c:199:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
# free_parrays(){
# ^~~~~~~~~~~~
# arrays.c:213:1: warning: control reaches end of non-void function [-Wreturn-type]
# }
# ^
# arrays.c:215:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
# make_parrays()
# ^~~~~~~~~~~~
# arrays.c:261:1: warning: control may reach end of non-void function [-Wreturn-type]
# }
# ^
#9 warnings generated.
# compiling compute.c
# compute.c:27:1: warning: 'extern' ignored on this declaration [-Wmissing-declarations]
# extern  struct  nf {
# ^
# compute.c:40:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
# comp_errors(aold,atarget,aerror,e,ce_e)
# ^~~~~~~~~~~
# compute.c:65:4: warning: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))'
#                         exit(1);
#                         ^
# compute.c:65:4: note: please include the header <stdlib.h> or explicitly provide a declaration for 'exit'
# compute.c:110:1: warning: control may reach end of non-void function [-Wreturn-type]
# }
# ^
# compute.c:113:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
# comp_deltas(apold,apnew,awt,adwt,aold,anew,aerror)
# ^~~~~~~~~~~
# compute.c:215:2: error: non-void function 'comp_deltas' should return a value [-Wreturn-type]
#         return;
#         ^
# compute.c:218:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
# comp_backprop(awt,adwt,aold,amem,atarget,aerror,local)
# ^~~~~~~~~~~~~
# compute.c:401:2: error: non-void function 'comp_backprop' should return a value [-Wreturn-type]
#         return;
#         ^
#6 warnings and 2 errors generated.
# make: *** [compute.o] Error 1

# make failed, exit code 2

# Gem files will remain installed in ~/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/tlearn-0.0.8 for inspection.
# Results logged to ~/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/extensions/x86_64-darwin-13/2.1.0-static/tlearn-0.0.8/gem_make.out