[PATCH 01/27] gdb/testsuite: Don't link with -lm on windows-msvc

When testing with --target=x86_64-pc-windows-msvc, all C/C++ tests
fail to build without this change, because target_compile adds -lm to
the link line, resulting in:

 lld-link: error: could not open 'm.lib': no such file or directory

x86_64-pc-windows-msvc targets the MSVC ABI, and uses the MSVC headers
and libraries, and there is no traditional math library in that
environment.  The math functions are part of the C runtime library
instead.

DejaGnu's default_target_compile does:

	if {[board_info $dest exists mathlib]} {
	    append extra_ldflags " [board_info $dest mathlib]"
	} else {
	    append extra_ldflags " -lm"
	}

Forcing people to use a custom board just to override mathlib would be
too heavy handed, IMNSHO.  Likewise for the approach of providing a
dummy empty m.lib library.  Instead, handle this by making gdb_compile
itself set the mathlib board config, when targeting windows-msvc.

Change-Id: Ib4671c9f207399f5ba437aaefb70373b6f298c41
---
 gdb/testsuite/lib/gdb.exp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 9b86d53be08..eea39d4e8cc 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -6553,6 +6553,25 @@ proc gdb_compile {source dest type options} {
     global objdir
     global gdb_saved_set_unbuffered_mode_obj
 
+    if {[istarget "*-*-windows-msvc*"]} {
+	# target_compile by default links with -lm, if the target
+	# board does not have 'mathlib' set to point to an alternative
+	# math library.  The windows-msvc target uses the MSVC headers
+	# and libraries, and there is no separate math lib in that
+	# case, the math functions are found in the C runtime library
+	# instead.  If mathlib is not set already in the target board,
+	# set it to empty and recurse, to override target_compile's
+	# default.
+	global board
+	set board [target_info name]
+	if {![board_info $board exists mathlib]} {
+	    set_board_info mathlib ""
+	    set result [gdb_compile $source $dest $type $options]
+	    unset_board_info mathlib
+	    return $result
+	}
+    }
+
     set outdir [file dirname $dest]
 
     # Handle compiling native Windows programs when testing Cygwin.
-- 
2.54.0