Compatibility fixes for musl.

--- a/m4/pc_from_ucontext.m4
+++ b/m4/pc_from_ucontext.m4
@@ -34,6 +34,7 @@ AC_DEFUN([AC_PC_FROM_UCONTEXT],
    pc_fields="$pc_fields uc_mcontext.arm_pc"           # Linux (arm arch 5)
    pc_fields="$pc_fields uc_mcontext.cr0_hi"           # Linux (e2k)
    pc_fields="$pc_fields uc_mcontext.gp_regs[[PT_NIP]]"  # Suse SLES 11 (ppc64)
+   pc_fields="$pc_fields uc_mcontext.gregs[[PT_NIP]]"
    pc_fields="$pc_fields uc_mcontext.mc_eip"           # FreeBSD (i386)
    pc_fields="$pc_fields uc_mcontext.mc_srr0"          # FreeBSD (powerpc, powerpc64)
    pc_fields="$pc_fields uc_mcontext.mc_rip"           # FreeBSD (x86_64 [untested])
@@ -77,7 +78,8 @@ AC_DEFUN([AC_PC_FROM_UCONTEXT],
                         pc_field_found=true)
        elif test "x$ac_cv_header_ucontext_h" = xyes; then
          AC_TRY_COMPILE([#define _GNU_SOURCE 1
-                         #include <ucontext.h>],
+                         #include <ucontext.h>
+                         #include <asm/ptrace.h>],
                         [ucontext_t u; return u.$pc_field == 0;],
                         AC_DEFINE_UNQUOTED(PC_FROM_UCONTEXT, $pc_field,
                                            How to access the PC from a struct ucontext)
--- a/src/getpc.h
+++ b/src/getpc.h
@@ -68,6 +68,9 @@
 typedef ucontext ucontext_t;
 #endif
 
+#if defined(__powerpc__) && !defined(PT_NIP)
+#define PT_NIP 32
+#endif
 
 // Take the example where function Foo() calls function Bar().  For
 // many architectures, Bar() is responsible for setting up and tearing
--- a/src/stacktrace_powerpc-linux-inl.h
+++ b/src/stacktrace_powerpc-linux-inl.h
@@ -186,7 +186,7 @@ static int GET_STACK_TRACE_OR_FRAMES {
           ucontext_t uc;
         // We don't care about the rest, since the IP value is at 'uc' field.
         } *sigframe = reinterpret_cast<signal_frame_64*>(current);
-        result[n] = (void*) sigframe->uc.uc_mcontext.gp_regs[PT_NIP];
+        result[n] = (void*) sigframe->uc.uc_mcontext.gp_regs[32];
       }
 #else
       if (sigtramp32_vdso && (sigtramp32_vdso == current->return_addr)) {
@@ -196,7 +196,7 @@ static int GET_STACK_TRACE_OR_FRAMES {
           mcontext_t mctx;
           // We don't care about the rest, since IP value is at 'mctx' field.
         } *sigframe = reinterpret_cast<signal_frame_32*>(current);
-        result[n] = (void*) sigframe->mctx.gregs[PT_NIP];
+        result[n] = (void*) sigframe->mctx.gregs[32];
       } else if (sigtramp32_rt_vdso && (sigtramp32_rt_vdso == current->return_addr)) {
         struct rt_signal_frame_32 {
           char dummy[64 + 16];
@@ -204,7 +204,11 @@ static int GET_STACK_TRACE_OR_FRAMES {
           ucontext_t uc;
           // We don't care about the rest, since IP value is at 'uc' field.A
         } *sigframe = reinterpret_cast<rt_signal_frame_32*>(current);
+#if defined(__GLIBC__)
         result[n] = (void*) sigframe->uc.uc_mcontext.uc_regs->gregs[PT_NIP];
+#else
+        result[n] = (void*) sigframe->uc.uc_mcontext.gregs[32];
+#endif
       }
 #endif
 
