[gimp] configure: work around a bug in AX_PROG_CC_FOR_BUILD...

[Jehan Pagès <jehanp src gnome org>]

commit dddcdb42c6d06cc9df331fdf2f3f87e4a11707f7
Author: Éric Hoffman <ehoffman videotron ca>
Date:   Wed Mar 29 14:30:36 2017 +0200

    configure: work around a bug in AX_PROG_CC_FOR_BUILD...
    
    ... when building on Windows.
    From bug 780270, comment 18:
    I'm still having issue with Windows MinGW, but I have traced the issue
    with the autoconf itself, and the autoconf-archive script
    "ax_prog_cc_for_build.m4". I have written to the autoconf-archive
    mailing list.
    
    It seem that this script never worked as intended since a long time
    because the way it works, it pushdef a few elements, then it disable
    cross-compiling (for the following test), and invoke AC_PROG_CC (which
    in turn invoke the code that find and set the exe extention). Then it
    grab the BUILD_EXEEXT from that. This is neat and simple, but the issue
    is that the autoconf AC_PROG_CC macro only invoke the code that is
    responsible for finding the exe (and obj) extensions once (with
    m4_expand_once). So, the end-result is that in the resulting configure
    script, EXEEXT is properly evaluated, but when comes the time to
    evaluate BUILD_EXEEXT, no test is performed to actually find the exe
    (and obj) extension, even if the cross-compilation option changed (which
    is the case for the duration of this test).
    
    So, BUILD_EXEEXT will always end up blank (defined, but blank).

 configure.ac |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index d0672d5..1aa0e2d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -233,6 +233,11 @@ AS_IF([test "x$0" != "x./configure"], [
 # Determine a C compiler to use
 AC_PROG_CC
 AX_PROG_CC_FOR_BUILD
+# Works around a bug in AX_PROG_CC_FOR_BUILD for build with MinGW on
+# Windows. See bug 780270, comment 18.
+case "$build_os" in
+  cygwin*|mingw32*|mingw64*)   BUILD_EXEEXT=.exe ;;
+esac
 AM_PROG_CC_C_O
 
 # Determine a C++ compiler to use